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

helium chronometry of apatite and titanite using nd yag laser heating we developed a technique for extracting 4he from apatite and titanite using laser heating that permits measurement of u and th and hence u th he ages on a single sample aliquot unlike direct laser heating quantitative he extraction and u th retention can be achieved when samples are indirectly laser heated to temperatures below their melting point resulting he ages are consistent with those obtained from conventional furnace heating of larger samples the major advantages of this technique include faster analysis and far lower more reproducible he blanks that permit single crystal analyses single crystal dating opens a variety of new avenues for u th he chronometry

range front fault scarps of the sierra el mayor baja california formed above an active low angle normal fault the canada david detachment a west dipping low angle normal fault juxtaposes pliocene and pleistocene sedimentary units over sheared bedrock in the sierra el mayor baja california along the west central range front the detachment is overlapped by quaternary alluvial fans that are in turn cut by fault scarps up to 7 m high map relationships suggest that scarp forming faults may sole into the detachment at very shallow depth 200 300 m palinspastic restoration of a topographic profile across 15 north striking scarps suggests that the ratio of net heave to net throw along scarp forming faults is 2 consistent with them soling into a low angle 30 fault these 15 scarps may have all formed in one earthquake with rupture propagation from depth to the near surface aided by clay gouge along the detachment the detachment is abandoned in the footwall east of the scarps

high pressure transformations in mgal2o4 x ray diffraction and transmission electron microscopy on laser heated diamond cell samples show that with increasing pressure mgal2o4 spinel transforms first to al2o3 corundum mgo periclase then to the cafe2o4 structured phase and finally to a new phase having the cati2o4 structure above 40 gpa the cafe2o4 and the cati2o4 structures are closely related and have almost the same densities and bulk moduli transformation from the cafe2o4 to the cati2o4 phase would be expected to take place in oceanic crust that is subducted deep into the lower mantle

early partial melting of diopside under high pressure abstract early partial melting epm in pyroxenes begins by the precipitation of tiny droplets 0 1 m or less of a molten phase highly enriched in silica at room pressure epm starts at temperatures markedly lower than the pyroxene solidus temperatures approximately 250 c below high pressure high temperature experiments up to 1 9 gpa and 1309 c were carried out in a piston cylinder apparatus on diopside single crystal bearing 1 2 wt feo to determine the influence of pressure on this phenomenon the systematic transmission electron microscopy investigation of the samples annealed at high t high p reveals that epm occurs up to p 1 5 gpa above 1208 c at po2 ranging from 2 0 10 17 and 2 5 10 16 mpa these t p conditions are well below the solidus temperature of pure camgsi2o6 1594 c at 1 5 gpa for pressures above 1 7 0 2 gpa no epm precipitates are found in sample annealed for 2 h 30 min at temperatures up to 1309 c

flow around a cylinder shallow water modeling with diffusion dispersion the equations for two dimensional 2d horizontal flow are obtained by vertical depth integration of the continuity and momentum equations diffusion dispersion terms appear containing turbulent and dispersion stresses turbulent stresses are expressed with the eddy viscosity concept dispersion stresses are evaluated using velocity distributions along and across the curved streamlines the differential equations are solved using the maccormack scheme the numerical simulation was done for three runs the expected alteration of the flow field around the cylinder is evident notably the wake behind the cylinder for comparison two similar runs were performed in a laboratory channel the velocity vectors upstream from the cylinder and along its sides are in reasonably good agreement however very close to the cylinder the simulation sometimes underpredicts the velocities downstream from the cylinder agreement is satisfactory both inside and outside the wake the flow depths at the centerline both upstream and downstream from the cylinder are also in good agreement however along the cylinder circumference the simulated flow depths are lower than the observed ones

mineral assemblages of basalt in the lower mantle high pressure x ray diffraction and analytical transmission electron microscopy of laser heated diamond cell samples show temperature dependent phase assemblages in mid ocean ridge basalt morb at lower mantle pressures the assemblage observed below 2250 k consists of only stishovite plus an orthorhombic perovskite phase space group pbnm or cmcm having a composition close to that of morb but slightly depleted in sio2 with increasing temperature the assemblage transforms to orthorhombic and cubic perovskites plus stishovite and then at 3250 k to orthorhombic and cubic perovskites plus stishovite and the cafe2o4 phase all of these assemblages could be present in slabs that have penetrated deep into the mantle and can therefore contribute to the seismological heterogeneity of the lower mantle

geophysical and isotopic mapping of preexisting crustal structures that influenced the location and development of the san jacinto fault zone southern california we examine the role of preexisting crustal structure within the peninsular ranges batholith on determining the location of the san jacinto fault zone by analysis of geophysical anomalies and initial strontium ratio data a 1000 km long boundary within the peninsular ranges batholith separating relatively mafic dense and magnetic rocks of the western peninsular ranges batholith from the more felsic less dense and weakly magnetic rocks of the eastern peninsular ranges batholith strikes north northwest toward the san jacinto fault zone modeling of the gravity and magnetic field anomalies caused by this boundary indicates that it extends to depths of at least 20 km the anomalies do not cross the san jacinto fault zone but instead trend northwesterly and coincide with the fault zone a 75 km long gradient in initial strontium ratios sr i in the eastern peninsular ranges batholith coincides with the san jacinto fault zone here rocks east of the fault are characterized by sr i greater than 0 706 indicating a source of largely continental crust sedimentary materials or different lithosphere we argue that the physical property contrast produced by the peninsular ranges batholith boundary provided a mechanically favorable path for the san jacinto fault zone bypassing the san gorgonio structural knot as slip was transferred from the san andreas fault 1 0 1 5 ma two historical m6 7 earthquakes may have nucleated along the peninsular ranges batholith discontinuity in san jacinto valley suggesting that peninsular ranges batholith crustal structure may continue to affect how strain is accommodated along the san jacinto fault zone

late cenozoic basin and range structure in western mexico adjacent to the gulf of california the area that surrounds the gulf of california was intensely faulted during the late cenozoic prior to opening of the gulf in a representative 10 000 km 2 area of southern sinaloa the faults consist of a predominant north northwest striking set having normal displacement and a subsidiary east northeast set having largely strike slip displacement two domains are recognized one in which both fault trends are abundant and one in which east northeast faults are minor in the former north northwest faults form a series of mostly southwest tilted half grabens filled with upper tertiary sediments displacement on individual faults is commonly as much as several kilometers east northeast faults probably represent accommodation zones between areas of differential extension in the second domain north northwest faults form an extensive graben system major faults dipping 40 to 70 into the graben and having several kilometers of cumulative displacement are spaced every 5 to 10 km bedding attitudes indicate that fault blocks are rotated as much as 65 dependent upon assumptions about subsurface geometry of the faults total extension may range from 20 to 50 fault geometry and stress orientations calculated from fault slip data indicate that the least principal stress was east northeast k ar ages of three north northwest striking dikes indicate that east northeast extension began as early as 32 ma ages of tilted volcanic rocks however indicate that measurable tilting and therefore most faulting began after about 17 ma faulting having similar timing style and orientation occurred throughout the area surrounding the gulf of california as far south as nayarit on the mexican mainland and as far north as areas in sonora that are unequivocally within the basin and range province this continuity and similarity in characteristics of faulting around the gulf to those of early basin and range faulting in the united states indicate that the area around the gulf is part of the basin and range province this basin and range faulting probably created the widely recognized but debated proto gulf of california the present gulf of california opened by ocean floor spreading and transform faulting in a zone already weakened by basin and range extension the lack of correspondence between areas of extension and likely areas of crustal thickening related either to laramide contraction or magmatism argues against a model of basin and range extension related solely to spreading of overthickened crust

a seismic refraction survey of the imperial valley region california the u s geological survey conducted an extensive seismic refraction survey in the imperial valley region of california in 1979 the imperial valley is located in the salton trough an active rift between the pacific and north american plates forty shots fired at seven shot points were recorded by 100 portable seismic instruments at typical spacing of 0 5 1 km more than 1300 recording locations were occupied and more than 3000 usable seismograms were obtained we analyzed five profiles using a standard ray tracing program constructed a contour map of reduced travel times from our most widely recorded shot point and modeled an existing gravity profile across the salton trough results are itemized 1 all models have in common a sedimentary layer vp 1 8 5 0 km s a transition zone vp 5 0 5 65 km s a basement vp 5 65 km s in the imperial valley 5 9 km s on the bordering mesas and subbasement vp 7 2 km s 2 the sedimentary layer ranges in thickness along the axis of the salton trough from 3 7 km salton sea to 4 8 km u s mexican border on the bordering mesas it is quite variable in thickness 3 the transition zone is about 1 km thick in most places in the imperial valley there are no marked velocity discontinuities in this zone between the sedimentary layer and basement on the bordering mesas however there is a discontinuity at the top of this zone 4 there are apparently two types of basement on the bordering mesas basement is crystalline igneous and metamorphic rocks in the imperial valley basement is mostly lower greenshist facies sedimentary rocks based primarily on the smooth transition in character from sediment to basement arrivals the low value of basement velocity and the fact that deep 4 km wells in the valley penetrate only the upper part of the known cenozoic stratigraphic column for the salton trough 5 the subbasement or intermediate crustal layer ranges in depth along the axis of the salton trough from 16 km salton sea to 10 km u s mexican border gravity modeling requires that this layer deepen and or pinch out beneath the bordering mesas and mountain ranges based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the imperial valley the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust 6 several structures are seen that affect basement transition zone and deeper parts of the sedimentary layer they include a scarp along the imperial fault as much as 1 km down to the northeast and a scarp passing roughly along the topographic boundary between the imperial valley and the bordering mesa to the west as much as 3 km down to the east we interpret the latter scarp to be the suture or rift boundary between the older crystalline basement on the mesa and the younger metasedimentary basement in the imperial valley 7 on a contour map of reduced travel time from our most widely recorded shot point subtle patches of early arrivals among otherwise late arrivals in the central imperial valley correlate well with known geothermal resource areas having reservoir temperatures of more than 150 c apparently the salton trough is a location where new crust is being generated as the rift opens mafic intrusive rocks fill it from below as sedimentary rocks fill it from above rifting and intrusion produce high heat flow that metamorphoses the sedimentary rocks to shallow depth metasedimentary basement in the imperial valley and thus consolidates the new crust

patterns of mineral transformations in clay gouge with examples from low angle normal fault rocks in the western usa abstract neoformed minerals in shallow fault rocks are increasingly recognized as key to the behavior of faults in the elasto frictional regime but neither the conditions nor the processes which wall rock is transformed into clay minerals are well understood yet understanding of these mineral transformations is required to predict the mechanical and seismogenic behavior of faults we therefore present a systematic study of clay gouge mineralogy from 30 outcrops of 17 low angle normal faults lanf s in the american cordillera to demonstrate the range and type of clay transformations in natural fault gouges the sampled faults juxtapose a wide and representative range of wall rock types including sedimentary metamorphic and igneous rocks under shallow crustal conditions clay mineral transformations were observed in all but one of 28 faults one fault contains only mechanically derived clay rich gouge which formed entirely by cataclasis clay mineral transformations observed in gouges show four general patterns 1 growth of authigenic 1m d illite either by transformation of fragmental 2m 1 illite or muscovite or growth after the dissolution of k feldspar illitization of fragmental illite smectite is observed in lanf gouges but is less common than reported from faults with sedimentary wall rocks 2 retrograde diagenesis of an early mechanically derived chlorite rich gouge to authigenic chlorite smectite and saponite mg rich tri octahedral smectite 3 reaction of mechanically derived chlorite rich gouges with mg rich fluids at low temperatures 50 150 c to produce localized lenses of one of two assemblages sepiolite saponite talc lizardite or palygorskite chlorite quartz and 4 growth of authigenic di octahedral smectite from alteration of acidic volcanic wall rocks these transformation groups are consistent with patterns observed in fault rocks elsewhere the main controls for the type of neoformed clay in gouge appear to be wall rock chemistry and fluid chemistry and temperatures in the range of 60 180 c

northern east pacific rise evolution from 25 m y b p to the present the northeast pacific topography and magnetic lineations 25 m y b p to the present record the traces of three major spreading reorganizations only one spreading center is observed today but there is evidence for several ephemeral episodes of twin spreading accompanying the evolution from an extensive pacific guadalupe plate boundary to a much shortened pacific cocos and pacific rivera plate boundary the 25 m y b p plate reorganization culminated with the formation of the guadalupe plate bound by the murray fracture zone to the north and the cocos nazca spreading ridge to the south between 25 and 12 5 m y b p spreading continued while the plate retained the same general outline the 12 5 11 m y b p reorganization resulted in the creation of a much shortened pacific cocos plate boundary located in its early stages over the mathematician seamounts and a much reduced cocos plate the last reorganizaton 6 5 3 5 m y b p resulted in the abandonment of the mathematician spreading ridge as a pacific cocos plate boundary in favor of the east pacific rise

the opening of the gulf of california near loreto baja california mexico from basin and range extension to transtensional tectonics abstract detailed structural field analyses of faults and fractures have led to the reconstruction of the tectonic evolution of the loreto region baja california mexico during the opening of the gulf of california three main tectonic events have been recognized i formation of large scale radial structures along the future main gulf escarpment here interpreted as associated with emplacement of magmatic bodies at shallow levels during the beginning of extension processes in early middle miocene ii ne sw extension basin and range event active since 15 ma producing nw se normal dip slip faults in the oligo miocene volcaniclastic basement and indicating high extension rates iii transtensional tectonics recorded especially in the plio quaternary sediments and associated with the activation of nw se dextral strike slip faults and n s normal faults the tectonic structures related to the basin and range extension are sealed by the plio quaternary sediments of the loreto basin and by the volcanics of the mencenares volcanic complex a huge volcanic field interfingering with the sedimentary succession of the basin

late mesozoic and cenozoic tectonic history of south central california the late mesozoic cenozoic history of south central california is punctuated by at least five major tectonic events the oldest of these involves late cretaceous drag folding and thrusting of epizonal shelf sediments westward over and to the north along the rising eastern margin of the peninsular ranges batholith these overthrusts with associated drag folds and nappes form imbricate stacks of metasediments slices of plutons and mylonites which overlie a regional core of cretaceous granitic plutons of the peninsular ranges batholith a second distinct kind of epizonal low temperature crustal displacement includes an episode of mid tertiary detachment faulting and folding this represents the westerly extension of the detachment terrane of eastern california and western arizona and utah superimposed upon the cretaceous structural features the mid tertiary motions involve major e w extension and n s compressive strain with formation of east trending antiformal core complexes remarkably parallel to a pervasive set of fold axes and b lineations formed in the cretaceous thrusting and folding all of these features are repeatedly segmented and displaced by step faulting as sial in the salton trough is fragmented and depressed 5 to 10 km below sea level at intervals throughout the cenozoic the irregular step faulted interface of the peninsular ranges batholith and salton trough is accentuated by a series of ne to se striking left lateral and dipslip faults with displacements of several to 10 kilometers these appear to both predate and occur during movements along the san andreas system pliocene pleistocene right lateral faulting a younger extension of the san andreas displacements north of the transverse ranges has broken and displaced all older structural features with total net slip in south central california of perhaps 300 km total right lateral slip along the several strands of the san andreas system are measurable via reconstruction of the mosaic of numerous older predominantly e w trending linear elements mid tertiary antiforms and cretaceous and older rocks right lateral strain on the san andreas appears to either alternate with or to be a byproduct of n s compressional strain from the pleistocene to the present a resultant effect is a large family of e w trending folds and fault scarps in plio pleistocene sediments

x ray microanalysis of high pressure high temperature phases synthesized from natural olivine in a diamond anvil cell the transformation products of natural mg0 83fe0 17 2sio4 olivine have been prepared at high pressures and high temperature in a laser heated diamond anvil cell dac the study of the quenched high pressure phases has been performed using analytical transmission electron microscopy the iron magnesium partition coefficient between perovskite pv and magnesiowustite mw defined as kmw pv fe mg mw fe mgpv was measured in samples synthesized at pressures from 25 to 75 gpa under various heating conditions in all cases iron goes preferentially into magnesiowustite it was observed however that the partition coefficient decreases strongly between 25 and 40 gpa at pressures higher than 40 gpa it is constant 3 5 within experimental error although there is a systematic increase in k as t decreases the effect of temperature is weak at high pressures from the values of the iron partition coefficient and previously existing thermodynamic values we obtain a consistent thermodynamic data set for the major constituents of the lower mantle the equilibrium compositions of mg fe 2sio4 spinel mg fe sio3 perovskite and mg fe o magnesiowustite phases have been measured and a new high pressure phase of composition mg fe si2o5 is shown to occur in very high temperature regions probably due to high temperature breakdown or non stoichiometry of mg fe sio3 perovskite

post 10 ma uplift and exhumation of the northern coast mountains british columbia apatite u th he ages reveal three distinct periods in the exhumation history of the northern coast mountains 54 n a well developed helium partial retention zone indicates little or no exhumation between ca 30 and 10 ma beginning at 10 ma and extending to at least 4 ma the range underwent steady but slow exhumation of 0 22 mm yr after which the exhumation rate increased by at least 70 an 85 km long he age traverse across the orogen at sea level shows that vertical offsets on post 10 ma faults are minor furthermore the sea level he ages 14 2 8 ma inversely correlate with local mean elevation along the traverse these data suggest that the coast mountains first appeared as a significant topographic feature only within the past few million years this history is not consistent with suggestions that uplift of the range resulted from formation of the queen charlotte basin in early middle miocene time we speculate that intense erosion by alpine and continental glaciation after 2 5 ma triggered isostatic uplift and creation of the modern topography of the coast mountains

elasticity of diopside to 8 gpa and 1073 k and implications for the upper mantle abstract simultaneous measurements of elastic wave velocities and density have been conducted on diopside by a combined ultrasonic interferometry and x ray diffraction methods at pressure and temperature conditions relevant to the earth s upper mantle the current study not only yielded the first direct measurement of the shear modulus at high pressures but also enabled a simultaneous determination of the bulk and shear properties and their pressure and temperature derivatives from the measured density and velocities finite strain analysis of the experimental data results in ks0 116 4 7 gpa k s 0 4 9 1 ks t p 0 012 1 gpa gpa g 0 1 6 1 and g t p 0 011 1 gpa k with these results and the same finite strain equations the p and s wave velocities of diopsidic mantle clinopyroxene were calculated along a 1600 k adiabatic geotherm in comparison with other mantle minerals the seismic velocities of diopsidic clinopyroxene at upper mantle depths are 1 3 higher than those of orthopyroxene 1 2 and 6 8 lower than those of olivine and majoritic garnet respectively

denudation of metamorphic core complexes and the reconstruction of the transition zone west central arizona constraints from apatite fission track thermochronology apatite fission track thermochronologic results from transects across the basin and range and transition zone provinces in west central arizona provide constraints on the denudational history and structural framework of the region apatite fission track ages decrease from 21 to 14 ma in the harcuvar mountains and from 16 to 13 ma in the buckskin rawhide mountains in the slip direction sw ne of detachment faults in the lower plates of the metamorphic core complexes mean lengths of confined fission tracks from the core complexes are all 14 m indicating that the apparent apatite ages record rapid cooling through the apatite partial annealing zone 40 c m y for lower plate rocks apparent apatite ages in the transition zone province generally increase from 25 ma to 100 ma away from the basin and range province this trend of increasing apatite age is disrupted by faulting as many as seven times at the fronts of major mountain ranges and within valleys between the weaver mountains and the colorado plateau gradients of apatite fission track age and confined track length with elevation in the mountain ranges and in the phillips kirkland drill hole reveal parts of denuded mesozoic and cenozoic apatite partial annealing zones these paleopartial annealing zone profiles provide a reference datum for preextension reconstructions of fault blocks the reconstructions indicate that the major faults in the transition zone province have relative displacements of 1 km and that offset on them occurred mostly after 25 ma these data also indicate that most of the rocks now exposed in the transition zone of west central arizona were not exposed until miocene time

major late cretaceous cooling events in the eastern peninsular ranges california and their implications for cordilleran tectonics fission track ages from the peninsular ranges batholith in the san jacinto mountains near palm springs california indicate two major late cretaceous cooling events concordant sphene and zircon fission track ages of plutonic rocks tonalite and low k granodiorite range from ca 85 ma to 98 ma with a mean of 92 ma the concordance of these ages over 2 5 km of exposed vertical section coupled with previous u pb zircon dating 94 93 ma implies major cooling from crystallization temperatures to below 235 c between 94 ma and ca 92 ma cooling is attributed to initial intrusion of the tonalite and granodiorite into an evolving brittle ductile normal fault zone detachment followed by 10 14 5 km of exhumation during continued crustal extension along the santa rosa mylonite belt field relationships microstructural fabrics and amphibole thermobarometry from the mylonite belt support the notion that extensional faulting was preceded by west directed thrusting this sequence of tectonic thickening followed by extension is similar to other areas in the southwestern cordillera and may represent the creation and failure of an unstable regional crustal welt in late cretaceous time a second event is evident from concordant fission track ages and indistinguishable tracklength distributions from apatite at all elevations these data indicate very rapid cooling of the tonalitic plutons through the apatite partial annealing zone 130 70 c between ca 74 ma and 80 ma mean age of 76 ma the occurrence of late cretaceous age regional unconformities throughout baja and southern california suggests that this cooling event was the result of surface uplift and erosion by using a reasonable range of geothermal gradients 20 40 c km the cooling data imply a minimum of 4 5 km of exhumation in conjunction with the ca 76 ma event surface uplift coincided with several important changes in the north american and farallon or kula plate motions these include an increase in convergence rate an increase in the absolute velocity of the north american plate and a change in the direction of convergence between the plates the changes in plate motions are thought to have initiated low angle or flat slab subduction that in turn was responsible for the laramide deformation of the cordilleran foreland surface uplift erosion and associated cooling during exhumation of the eastern peninsular ranges are considered to be an initial expression of laramide tectonism phenomena documented here are consistent with uplift brought on by tectonic erosion of the north american lithospheric mantle due to low angle subduction and by subsequent underplating of lower crustal rocks

structural and tectonic evolution of western cuba fold and thrust belt 1 we present balanced and restored cross sections across the western cuba fold and thrust belt to illustrate its structure and tectonic evolution based on the interpretation of multichannel seismic profiles field data and balancing techniques the nnw sse trending soroa cross section intersects from south to north 1 the los palacios basin 2 the sierra del rosario antiformal stack 3 the bahia honda thrust unit 4 the frontal 3 km thick imbricated thrust sheets and 5 the foredeep basin at depth a duplex that links the sierra del rosario unit and the frontal imbricates is defined using the thickness of the cover thrust sheets of the imbricated north american margin the minimum calculated shortening is of 130 km in a sse or south direction to fit large scale plate tectonic reconstructions however a more allochthonous alternative using thinner cover sequences is also discussed which restores the bahia honda unit 220 km to the south of its present position syntectonic deposits in combination with a forward kinematic model provide a good constraint for the tectonic evolution of the orogen the internally deformed bahia honda unit overthrusted the north america margin cover successions during danian and selandian times 65 5 60 ma before the stacking of these cover rocks during selandian and thanetian times 60 56 ma the growth of a large antiformal stack beneath the sierra del rosario occurred at early middle eocene 56 45 ma generating about 4 km of structural relief between the paleoisland of cuba and the synorogenic seafloor before the final infill of the foredeep

sonora mexico source for the eocene poway conglomerate of southern california alluvial fan conglomerates of the eocene poway group are composed largely of exotic rhyolite and dacite clasts derived from far to the east of their eocene depositional site remnants of the upper jurassic bedrock source of the poway rhyolite clasts may yet be exposed in hills in sonora mexico for this study pieces of bedrock were taken from hills 13 km west of el plomo in sonora clasts texturally and mineralogically similar to the sonoran bedrock were collected from the apex of the eocene alluvial fan in san diego county california nine couplets of bedrock and conglomerate clast samples textural twins were analyzed for 16 trace elements selected for their wide range of behaviors during magmatic and alteration processes statistical comparisons of the trace element data by using the standard error of the difference method show that there are no significant differences between the two populations these data strongly suggest that the rhyolitic bedrock hills west of el plomo were part of the source terrane for the eocene conglomerate in san diego the latitudinal separation between bedrock source and the site of deposition is only the 2 created by the opening of the gulf of california this implies that any boundary separating a paleomagnetically efined baja borderland terrane from the craton since eocene time was at least 100 km east of the gulf of california in northernmost sonora

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

quaternary alluvial fan development climate and morphologic dating of fault scarps in laguna salada baja california mexico article i nfo late quaternary slip across the canada david detachment has produced an extensive array of quaternary scarps cutting alluvial fans along nearly the entire length 60 km of the range bounding detachment eight regional alluvial fan surfaces q1 youngest to q8 oldest are defined and mapped along the entire sierra el mayor range front terrestrial cosmogenic nuclide 10 be concentrations from individual boulders on alluvial fan surfaces q4 and q7 yield surface exposure ages of 15 5 2 2 ka and 204 11 ka respectively formation of the fans is probably tectonic but their evolution is strongly moderated by climate with surfaces developing as the hydrological conditions have changed in response to climate change on milankovitch timescales systematic mapping reveals that the fault scarp array along active range bounding faults in sierras cucapa and el mayor can be divided into individual rupture zones based on cross cutting relationships with alluvial fans quantitative morphological ages of the laguna salada fault scarps derived from linear diffusive degradation modeling are consistent with the age of the scarps based on cross cutting relationships the weighted means of the maximum mass diffusivity constant for all scarps with offsets b 4 m is 0 051 and 0 066 m 2 ka for the infinite and finite slope solutions of the diffusion equation respectively this estimate is approximately an order of magnitude smaller than the lowest diffusivity constants documented in other regions and it probably reflects the extreme aridity and other microclimatic conditions that characterize the eastern margin of laguna salada

melting of pyrope mg3al2si3o12 up to 10 gpa possibility of a pressure induced structural change in pyrope melt the melting of pyrope mg3al2si3o12 has been determined experimentally at pressures up to 10 gpa pyrope melts congruently in the pressure range 3 5 10 gpa at pressures below 7 gpa aluminous enstatite of pyrope composition crystallizes during quenching whereas quench pyrope appears at higher pressures moreover the slope of the melting curve of pyrope decreases significantly at pressures above 7 gpa these changes in both the nature of the quench phase and the slope of the melting curve suggest a pressure induced coordination change for al3 in pyrope melt at around 7 gpa this pressure induced coordination change of al3 probably occurs even in magmatic liquids generated in the deep upper mantle and should affect some of their physical properties

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

thermal evolution of monte blanco dome low angle normal faulting during gulf of california rifting and late eocene denudation of the eastern peninsular ranges footwall rocks of the canada david detachment fault northeastern baja california record late eocene earliest oligocene and late neogene cooling events previously unrecognized in the region biotite 40 ar 39 ar ages of 65 ma reflect slow cooling through 350 c 5 to 10 m y later than is typical in the region multiple diffusion domain modeling of k feldspar 40 ar 39 ar release spectra shows very slow cooling 1 c m y from 65 to 45 ma accelerated cooling from 315 c to 215 c between 45 and 33 ma records at least 3 4 km of denudation that we relate to east side up reactivation of late cretaceous structures that generally follow the oceanic continental suture previously established rivers flowing west from mainland mexico apparently became further entrenched during this uplift and continued to supply distinctive rhyolitic detritus to the coast ultimately surface uplift disrupted and rerouted the extraregional rivers some 2 to 6 m y before the cooling event ended footwall rocks remained nearly isothermal from 30 to 15 10 ma when renewed rapid cooling 33 17 c m y began in response to footwall exhumation by top to the west low angle normal faulting that accommodated rift related extension in what finally became the gulf of california apatite fission track and u th he ages of 5 ma and 4 ma respectively record final detachment related cooling through 110 c to 70 c thermal kinematic modeling suggests that 5 7 km of late neogene tectonic denudation and 10 12 km of horizontal extension were necessary in order to unroof the samples by 2 ma additional extension of unknown magnitude has probably occurred subsequently geodetically measured horizontal extension rates are considerably higher than the long term extension rate that can be attributed to detachment faulting

the yaqui ridge antiform and detachment fault mid cenozoic extensional terrane west of the san andreas fault the narrow zone of mid cenozoic detachment terrane in the southwestern united states can be widened and traced across the southern strands of the san andreas fault zone into the eastern margin of the peninsular ranges batholith the yaqui ridge core complex and detachment fault in southern borrego valley california exemplifies the nature of detached terranes in southcentral california the detachment fault dips 10 40 to the northeast and separates a lower core of gneissic late cretaceous granodiorite from an unconsolidated unmetamorphosed megabreccia of eocene to miocene age foliations in the lower plate generally conform to the strike and dip of the overlying detachment fault and become less distinct away from the fault the megabreccia which forms the upper plate is composed of unsorted fairly well rounded clasts characteristic of batholithic and metasedimentary rocks of the region plio pleistocene lacustrine sediments unconformably overlie the megabreccia in some areas the detachment fault and upper plate are expressed as a series of klippen which parallel yaqui ridge along the northeast and southeast flanks the fault is marked by a typically narrow 6 10 cm band of intensely sheared cataclasite a chlorite breccia zone occurs below the cataclasite and gradually grades into the less deformed regionally foliated gneissic granodiorite on the southeast flank what appears to be an extension of the cataclasite and detached upper plate is exposed only locally near the southeast nose of the antiform here the upper plate consists of blocks of pale gray to tan aphanitic fault gouge superimposed upon and cutting through the detachment related features is a broad zone of left oblique slip faulting termed the yaqui ridge shear zone this fault zone trends wnw ese parallel to yaqui ridge and separates the detachment fault from the ridge merging with the detachment fault near the eastern margin further east the shear zone widens into a massive zone of gouge 400 m wide and disappears under the sediments of borrego valley and the salton trough to the west the shear zone slices into the eastern margin of the peninsular ranges batholith and extends the marginal plutons in an e w direction four dominant tectonic episodes can be recognized from exposures at and adjacent to yaqui ridge 1 a late cretaceous synkinematic cataclasis and metamorphism accompanying and following emplacement of the yaqui ridge and related plutons and forming a pervasive regional nw trending foliation and ne trending lineation 2 a subsequent mid cenozoic shallow low temperature event of detachment faulting and folding which produces localized brittle cataclasis which overprints the earlier nw trending fabrics 3 the late miocene early pliocene development of yaqui ridge shear zone along the northeast margin of yaqui ridge shearing rotating and overprinting the fabrics formed during the late cretaceous plutonism and regional deformation 4 a pleistocene episode of high angle faulting and folding which produce a series of e w trending anticlines and synclines in upper pliocene to pleistocene sediments linear elements which have formed throughout the several late cretaceous to cenezoic deformations are concordant

evolution of the margin of the gulf of california near loreto baja california peninsula mexico the gulf of california is a prime example of a young oblique divergent plate boundary this type of plate boundary is much less well understood than classic rifts and passive margins in divergent plate settings a complexity in the gulf of california is that the modern oblique rifting was preceded by a stage of orthogonal rifting we have used extensive mapping as well as stratigraphic geomorphic and structural analysis to determine the history of deformation near loreto on the southern baja california peninsula during formation of the gulf of california our data support the suggestion that middle to late miocene protogulf stage 12 to ca 6 ma orthogonal rifting was overprinted by transtensional structures during pliocene to quaternary time though we cannot closely define the time of change we further demonstrate that the plate margin was structurally segmented in the protogulf stage and then complexly overprinted by transtensional structures that suggest one type of process for initiating transform faults the loreto segment is 85 km long along the rift and is bounded on the west by a discontinuous series of aligned down tothe east extensional monoclines and normal faults these structures lie along or as far as 4 km in front east of a steep 1000 1600 m high main gulf escarpment that defines the western topographic margin of the rift zone the loreto segment is further defined by a rise in elevation of the main gulf escarpment of up to 1 km and an 500 800 m increase in structural relief of prerift strata in the escarpment from the segment boundaries to the center of the segment structural analysis of secondary faults of the loreto segment shows that fault populations that are known to be pliocene to quaternary in age are mixed normal and dextral normal faults with a bulk extension direction of west northwest eastsoutheast 2808 1008 in contrast fault populations that cut only prerift oligocene middle miocene volcanic and sedimentary rocks are mainly normal faults with northeast southwest to east northeast west southwest bulk extension directions that average 2458 658 these faults are interpreted to be late miocene in age and formed during protogulf orthogonal rifting thus the pliocene faults record a major change of extension direction of 358 from latest miocene to pliocene time compatible with the onset of oblique rifting in the gulf of california oblique divergent overprinting is mainly expressed as the loreto fault and loreto basin in the northern part of the segment the loreto fault and basin evolved with major fault reorganizations and partial basin inversion many observations suggest that the pliocene loreto fault was linked to two nascent transform faults in the gulf and that carmen island rotated 358 408 clockwise within the complex fault system

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

modeling of the temperature sensitivity of the apatite u th he thermochronometer apatite u th he apparent ages will generally reflect residence for extended periods at temperatures where helium is neither quantitatively retained nor lost by diffusion to characterize the response of apatite he ages to thermal histories involving partial he retention we explored solutions to the he production diffusion equation under thermally static conditions the analytical solution to this equation coupled with published diffusivity data demonstrates that the zone of partial he retention extends from about 40 c to 85 c this zone lies at temperatures 35 c cooler than the analogous fission track partial annealing zone he ages within the partial retention zone ultimately achieve a balance between he production and loss yielding a steady state age both the ultimate age and the time it takes to achieve this age are temperature dependent for example an apatite held at 75 c equilibrates to an age of 2 ma after 17 myr regardless of whether equilibrium is approached from a higher or a lower initial he age for representative dynamic thermal histories we evaluated apatite he ages using a numerical solution to the ingrowth diffusion equation the results illustrate the sensitivity of he ages to various geologic histories and are useful for understanding he age elevation relationships and for testing time temperature paths derived from apatite fission track length distributions in addition although he diffuses rapidly from apatite at shallow crustal temperatures modeling of ambient temperature fluctuations indicates that he ages are nearly unaffected by surficial processes

evidence for late cretaceous crustal thinning in the santa rosa mylonite zone southern california the santa rosa mylonite zone is part of a 100 km long belt of deformation along the eastern margin of the northern peninsular ranges batholith southern california in the northern santa rosa mountains complexly deformed mylonitic plutonic and metasedimentary tectonites are cut by a stacked sequence of five low angle faults evidence from field relationships and the analysis of minor structures microstructures and textures preferred orientation suggest that the coaxial component of deformation pure shear dominated over a noncoaxial simple shear component it is proposed that mylonitic deformation and low angle faulting which appear to be kinematically associated are the result of late cretaceous extension and thinning of the continental margin

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

a simple model of the 1982 83 californian el nino steric height steric height anomaly sea level and sea level anomaly show that the 1940 41 and the 1982 83 californian el nino events were resonant responses of the california current to the expansion and intensification of the aleutian low and decrease in strength of the north pacific high in both cases the subsurface anomalies were dynamically produced by onshore transport of subarctic water and a downward tilt of the inshore thermocline sea surface temperature anomalies however resulted from thermodynamic as well as dynamic processes most previous californian el nino or anti el nino events probably were resonant or anti resonant responses of the seasonal cycle of the california current to anomalous large scale atmospheric forcing

assessment of u th he thermochronometry the low temperature history of the san jacinto mountains california u th he ages have been measured on igneous apatites from the san jacinto mountains a high region at the junction of the peninsular and transverse ranges to investigate the potential of this technique for thermochronometry of slowly cooled rocks helium ages from 79 to 17 ma are younger than ages obtained by other dating techniques including apatite fission track counting and are consistent with laboratory experiments that indicate this system has a uniquely low closure temperature helium ages are strongly correlated with elevation and record the latest low temperature thermal evolution of the range they suggest relative tectonic quiescence in the latest cretaceous through mid tertiary and provide no evidence for rapid unroofing of the block during this period nor do they obviously require a large degree of uplift associated with convergence between the transverse and peninsular ranges in the last few million years helium ages document modest westward tilting of the block 7 and a significant vertical component of motion on the block s bounding faults after helium retention began this work suggests that apatite helium ages record low temperature tectonic and thermal histories that are not apparent from other dating techniques

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

high temperature deformation of diopside single crystal 1 mechanical data laboratory deformation experiments were carried out on diopside single crystals creep tests were made in a dead load apparatus at temperatures t 1020 to 1320 c axial compressive stresses 50 to 170 mpa with strain rates e ranging from 2 10 9 s 1 to 4 10 7 s 1 the specimens were oriented such that mechanical twinning was not possible the experiments were designed to activate 110 1 2 a b slip systems our results demonstrate that they are the major operative systems for t 1000 c and appear to be predominant over the 100 c slip system the mechanical data are fitted to a power law e a n exp e rt an inversion method was used to determine the parameter in a the activation energy e and the stress exponent n below a critical temperature tc 1130 1140 c e is found to be 440 30 kj mol associated with the 110 1 2 a b slip systems activated symmetrically without any contribution from the 100 c system orientation 2 e is found equal to 740 30 kj mol if all three systems have the same resolved shear stresses orientation 1 for t tc values of e drop to 50 15 and 85 30 kj mol for orientations 2 and 1 respectively so that the creep law of diopside becomes nearly temperature independent the law derived from the orientation 2 crystals could be consistent with natural deformations for this orientation and within the range of temperatures covered n is 6 5 0 4 the strong decrease in the activation energy at tc may be ascribed unambiguously to the occurrence of partial melting leading to microdroplets which pin the mobile dislocations ingrin et al this issue the high value found for the stress exponent n in the creep law suggests that clinopyroxene is softer than olivine at high stresses at t 900 c stress 20 mpa and e 10 15 s 1 while olivine is softer at lower stresses this competence inversion is predicted to occur within the ranges of stresses and strain rates expected in the lower crust and upper mantle

discussion of two dimensional solution for straight and meandering overbank flows by d alan ervine k babaeyan koopaei and robert h j sellin

high pressure and high temperature phase relations in diopside camgsi2o6 abstract phase transformations in diopside ca 0 92 mg 1 08 si 2 o 6 were studied by quench experiments at pressures of 18 24 gpa and temperatures of 1000 2400 k using an ma8 type multi anvil apparatus at moderate temperatures diopside decomposed into the assemblage of two components one with mgsio 3 stoichiometry and the other with casio 3 stoichiometry the mgsio 3 component crystallized into mg 2 sio 4 stishovite then transformed into mg 2 sio 4 stishovite mgsio 3 ilmenite and mgsio 3 perovskite with increasing pressure the casio 3 component was observed always as a glass in the recovered samples it is likely that casio 3 formed the cubic perovskite type structure under pressure at higher temperature diopside transformed into mg ca sio 3 garnet and casio 3 perovskite and then to mgsio 3 and casio 3 perovskites the solubility of ca into mg ca sio 3 garnet decreases with increasing pressure the stability field of ca bearing garnet is expanded to lower temperature range compared to that of pure mgsio 3 garnet the present results clarify the phase relations of diopside to the uppermost part of the lower mantle

late cenozoic extension of northeastern baja california mexico the pattern and history of normal faulting in a portion of northeastern baja california strongly suggest that the area developed as a result of east west crustal extension during late cenozoic time geometric and kinematic data strongly suggest that this area developed in concert with the opening of the proto gulf of california and the distension of the basin and range province in addition these data coupled with regional tectonic timing relations preclude the application of an oblique rifting model to this area two intervals of extensional tectonics have affected the region since the early miocene at present it is not known if strain was continuous or discontinous with respect to time the first and most intense period initiated between 17 and 9 m y ago was east west directed as determined from tilt patterns orientation of intra terrane transcurrent faults and fault plane kinematic indicators this deformation was dominated by high angle normal faulting and resulted in the disruption of a region wide low relief erosion surface new patterns of sedimentation were established and controlled by north trending grabens that filled with alluvial fan and marine deposits sporadic silicic eruptions occurred in conjunction with the extension thus allowing resolution of the complex history of tilting that accompanied normal faulting tilting and antithetic faulting resulted from the volume compensation required by the downward flattening of normal faults and by fault related monoclinal flexing and reverse drag effects extension during this interval was complete or substantially reduced by 6 m y b p the second interval of east west extension younger than 6 m y b p has been much less intense than the earlier event yet it seems to have utilized the same structures silicic volcanism also waned during this interval giving way to mafic eruptions related to rifting of the present gulf of california

early pleistocene initiation of the san felipe fault zone sw salton trough during reorganization of the san andreas fault system structural and stratigraphic analyses along the western margin of the salton trough show that the san andreas fault system was reorganized in early pleistocene time from a system dominated by two fault zones the san andreas fault and the west salton detachment fault to a network of dextral faults that include the san andreas and at least four dextral faults to the southwest the san felipe fault zone one of these dextral faults has 5 8 2 8 km of right separation and consists of three principal faults in the peninsular ranges these are the san felipe fault in the wnw sunset fault in the middle and fish creek mountains fault in the ese they form a left stepping array and bound domains in which the sunset conglomerate the older west salton detachment fault and cretaceous mylonitic rocks below the detachment are folded about wnw trending folds a complex flower structure within the left stepovers probably produced this fault parallel folding because all the rocks within stepovers of the san felipe fault zone from cretaceous to pleistocene are deformed about wnw trending folds and record broadly similar shortening strains we infer a quaternary age of deformation parts of the san felipe fault zone cut latest pleistocene to holocene surficial deposits and the fault zone is likely active evidence for early pleistocene initiation of the san felipe fault zone is preserved in conglomerate ne of the sunset fault poorly sorted angular boulder conglomerate and pebbly sandstone of the sunset conglomerate are 600 m thick and lie in angular unconformity on the pliocene palm spring group the conglomerate coarsens upward and toward the fault and is dominated by plutonic clasts derived from sw of it conglomerate beds contain up to 10 sandstone clasts recycled from older basin fill and accumulated in proximal to medial alluvial fans that were shed to the ne from uplifted rocks along the then active sunset fault based on lithologic stratigraphic structural and compositional similarities we correlate the sunset conglomerate to the pleistocene ocotillo formation clasts of recycled sandstone record erosion of detachment related basin fill that predates the san felipe fault and once covered the vallecito and fish creek mountains these crystalline cored mountain ranges first emerged from beneath basin fill during early slip above the nascent san felipe fault ca 1 1 1 3 ma later the san felipe fault zone cut upward folded cut across and deactivated the west salton detachment fault within a 9 km wide contractional bend and pair of left steps areas that accumulated sediment within this step over zone between ca 1 1 and ca 0 6 ma are currently being inverted and folded initiation of the san felipe fault in early pleistocene time was a significant event in the reorganization of the southern san andreas fault system the quaternary dextral faults broadened the plate boundary zone south westward from roughly 25 km during coeval slip on the san andreas fault and west salton detachment fault to 50 70 km and mark a change in the dominant structural style from transtension to distributed dextral faulting south of the big bend

late miocene to recent transtensional tectonics in the sierra san fermin northeastern baja california mexico basins and ranges within part of the gulf of california extensional province mexico have experienced complex distributed deformation including normal and strike slip faulting and block rotations linked to dextral shear at the pacific north america plate boundary in the sierra san fermin and southern sierra san felipe northeastern baja california normal faulting began between 12 5 and 6 ma although most extension occurred between about 6 and 3 ma strongly influencing thickness and distribution of ash flow tuffs and sedimentary deposits extension is generally 10 in 6 ma rocks and somewhat more in 12 5 ma rocks inversion of kinematic data interpreted together with published paleomagnetic data suggests that the axis of least principal stress was oriented between w e and sw ne in late miocene time our data indicate an important change in the amount of dextral shear but not necessarily the least principal stress direction wsw ene at about 3 ma structural constraints limit significant sinistral strike slip faulting conjugate to the dextral plate boundary to the last 3 my progressive changes in the geometry of faulting through time are consistent with regional strain partitioning within the pacific north america plate boundary zone and are predicted by physical and analytical models of oblique divergence as the orientation of the stretching vector changes to lower and lower values

geologic structural and thermochronologic constraints on the tectonic evolution of the sierra mazatan core complex sonora mexico new insights into metamorphic core complex formation 1 the sierra mazatan in northwestern mexico is the southernmost metamorphic core complex in the north american cordillera large magnitude tertiary extension at sierra mazatan involved both ductile and brittle slip along a major normal fault that presently dips 10 15 west extension was polyphase and involved an early period of extension from 25 to 23 ma followed by major slip from 21 to 16 ma total slip was 20 km and occurred at rates of 3 4 mm a this extension predated the plate boundary change to transtension at 12 ma and was largely decoupled from relative pacific north american plate motion numerous lines of evidence suggest that the presently low angle normal fault initiated at a steep dip 50 60 and was rotated to lower angles during slip when corrected for this tilting fault corrugations at sierra mazatan had a similar geometry to the segmentation of many active normal faults which is compatible with their origin as primary fault features many aspects of the sierra mazatan are comparable to large active normal faults indicating that this core complex formed owing to prolonged extension on an otherwise typical high angle normal fault therefore core complexes need not represent a fundamentally unique mode of crustal extension

magmatic arc asymmetry and distribution of anomalous plutonic belts in the batholiths of california effects of assimilation crustal thickness and depth of crystallization in order to better understand geologic fac tors controlling pronounced regional variations in whole rock chemistry mineralogy and mineral chemistry in the batholiths of california we calculate the magmatic intensive variables f hf f h 2 o f hf f hcl and f o 2 regional scale west to east increases in f oh in mafic silicates corresponding to the systematic i wc to i mc and i sc progression reflect orders of magnitude increase in f hf f h 2 o attending pluton crystallization low f hf f h 2 o of formation of western i wc types is consistent with their derivation from low fluorine source rocks in subducted oceanic slabs or the upper mantle in contrast higher f hf f h 2 o of crystallization of i mc and i sc types to the east implies the involvement of 1 progressively greater amounts of continental crustal source material such as biotite bearing metamorphic rocks their unweathered sedimentary derivatives retaining f rich mafic minerals or their fusion products and or 2 source materials which become more f rich toward the continental interior the regional distribution of i mc and i sc types suggests that the precambrian craton of western north america or derivative sediments may extend farther north in california and be morphologically more complex than previously thought from seemingly out of place occurrences of i wc plutons on the eastern slopes of the sierra nevada batholith we infer the existence of regions where precambrian basement was thin or absent in mesozoic time which prevented extensive cratonal contamination of subducted slab or upper mantle derived magmas new methods for estimating t f o 2 relations in the magmas demonstrate that i scr granites crystallize at oxygen fugacities as much as five orders of magnitude lower than those of i wc i mc and i sc types under conditions at or below the maximum stability limit of graphite in equilibrium with a c o h s gas phase the local scale formation of i scr granites in plutonic belts within specific wall rock terranes containing highly reducing sediments or metasediments may occur by contamination of i types with graphitic pelite or in some cases by the direct fusion of this reducing pelitic wall rock the spatial distribution of i scr granite provinces therefore is controlled simply by wall rock lithology amphibole geobarometry demonstrates a general west to east decrease in crystallization pressure across the sierra nevada batholith in contrast the peninsular ranges batholith displays a west to east crystallization pressure increase the bulk of the california batholiths crystallized at pressures less than 4 5 kb and depths less than about 15 19 km in the southern sierra nevada batholith the san gabriels and the eastern peninsular ranges however plutons crystallized at pressures exceeding 6 kb at deep crustal levels 23 km reconstruction of the pre erosion top of the batholith shows that in an east west cross section the central sierra nevada batholith was a horizontal tabular body with an aspect ratio of at least five to one

composition of the upper mantle geophysical tests of two petrological models the elastic properties of candidate mantle phases are used to test the viability of olivine rich pyrolitic and cao al 2o 3 rich eclogitic assemblages for the mantle high temperature adiabats for each phase of interest are constructed and compared to mantle seismic properties both pyrolitic and eclogitic assemblages satisfy the seismic properties between 200 and 400 km between 400 and 670 km depth an eclogitic assemblage yields a superior match to velocities and velocity gradients the 400 km seismic discontinuity may represent a chemical boundary between pyrolite and picritic eclogite piclogite or phase transformations in the olivine orthopyroxene components of a piclogitic assemblage containing about 16 olivine high velocity gradients in the transition zone may be explained by the transformation of ca rich cpx to majorite garnet seismic properties at the top of the lower mantle are consistent with pyrolite piclogite or perovskite implying that the 670 km discontinuity may be a chemical boundary

a reappraisal of seafloor spreading lineations in the gulf of california implications for the transfer of baja california to the pacific plate and estimates of pacific north america motion an analysis of seafloor spreading lineations in the southern gulf of california demonstrates that divergence of the baja peninsula from north america since 3 6 ma has been significantly slower than pacific north america pa na motion estimated solely from closures of global plate circuits this discrepancy remains unchanged or increases with modifications to the plate circuits and kinematic data used to predict pa na motion suggesting that it arises from the probably incorrect assumption that baja california has been rigidly coupled to the pacific plate since 3 6 ma a 15 increase in seafloor spreading rates in the southern gulf after 1 ma could have been caused by a transfer of motion along faults west of baja california to the modern day plate boundary or alternatively could indicate a recent speedup in pa na motion both interpretations imply that pa na motion is now 4 2 mm yr 1 2 faster than predicted by the 3 16 myr average nuvel 1a model

cenozoic tectonic evolution of the white mountains california and nevada the white mountains represent the westernmost range of the central northern basin and range province they are situated to the east of the unextended sierra nevada and represent a crustal block that is bounded along its western flank by the high angle white mountains fault zone the fault zone accommodates up to 8 km of total dip slip displacement investigation of the structural and thermal history of the white mountains indicates a two stage cenozoic tectonic evolution preextensional miocene volcanic rocks preserved along the eastern side of the range unconformably overlie mesozoic granitic basement and currently dip up to 25 to the east recording the total cenozoic tilt of the crustal block apatite fission track and u th he thermochronological data indicate that the white mountains underwent rapid exhumation and eastward tilting in the middle miocene starting at ca 12 ma geologic mapping 1 10 000 fault kinematic analysis and dating of younger volcanic sequences show that following middle miocene east west extension the white mountains have been dominated by right lateral transtensional deformation related to the walker lane belt the eruption of late miocene and pliocene volcanic sequences in the eastern white mountains postdates the majority of the uplift of the range as evidenced by infilling of paleodrainages and the presence of east directed flow fabrics fault kinematic indicators from the white mountains fault zone are characterized by apparent overprinting of dip slip fault motion indicators by right lateral slickenfibers and fault striations demonstrating that the range bounding fault system along the western side of the white mountains was reactivated as a dextral strike slip fault system at the northern and southern ends of the range pliocene right lateral transtension along this northwest southeast trending fault systems resulted in the formation of northeast trending pull apart basins that truncate the mountain range and transfer strike slip displacement eastward from the owens valley fault zone to the fish lake valley fault zone the inception of strike slip faulting in fish lake valley occurred at ca 6 ma as constrained by late miocene volcanic units right lateral faulting on the western side of the white mountains occurred at ca 3 ma and is distinctly younger than the faulting in the fish lake valley area indicating a westward migration of transcurrent deformation through time

phase transformations in diopside camgsi2o6 at pressures up to 25 gpa phase transformations in diopside have been studied at pressures up to 25 gpa and at temperatures of 1100 and 1500 c by a multiple anvil apparatus the following phase assemblages are found to be stable at given pressures diopside 15 and 18 gpa mg2sio4 spinel stishovite casio3 perovskite 19 and 20 gpa mgsio3 ilmenite casio3 perovskite 22 gpa and mgsio3 perovskite casio3 perovskite 24 5 gpa electron microprobe analysis reveals that the solubility of the mgsio3 molecule in casio3 cubic perovskite is limited to between 2 5 the solubility of the casio3 molecule in mgsio3 orthorhombic perovskite is more restricted being of the order of 2 or less these results are contradictory to earlier work in which diopside is reported to transform into a cubic perovskite structure of the same composition above 22 23 gpa

extensional segmentation of the main gulf escarpment mexico and united states periodic reversal of upper plate transport direction on miocene and younger extensional fault systems occurs in the northwestern gulf extensional province baja california and adjacent united states with segments in which the peninsular ranges are in either footwall or hanging wall positions accommodation zones separate these segments and are defined by changes of structural style and topography in the peninsular ranges and or the gulf extensional province these and other characteristics allow speculative definition of segments 50 150 km long along the baja california peninsula

response of deformation patterns to reorganization of the southern san andreas fault system since ca 1 5 ma abstract between 1 5 and 1 1 ma the southern san andreas fault system underwent a major reorganization that included initiation of the san jacinto fault zone and termination of slip on the extensional west salton detachment fault the southern san andreas fault itself has also evolved since this time with several shifts in activity among fault strands within san gorgonio pass we use three dimensional mechanical boundary element method models to investigate the impact of these changes to the fault network on deformation patterns a series of snapshot models of the succession of active fault geometries explore the role of fault interaction and tectonic loading in abandonment of the west salton detachment fault initiation of the san jacinto fault zone and shifts in activity of the san andreas fault interpreted changes to uplift patterns are well matched by model results these results support the idea that initiation and growth of the san jacinto fault zone led to increased uplift rates in the san gabriel mountains and decreased uplift rates in the san bernardino mountains comparison of model results for vertical axis rotation to data from paleomagnetic studies reveals a good match to local rotation patterns in the mecca hills and borrego badlands we explore the mechanical efficiency at each step in the modeled fault evolution and find an overall trend toward increased efficiency through time strain energy density patterns are used to identify regions of incipient faulting and support the notion of north to south propagation of the san jacinto fault during its initiation

chronological constraints on the thermal and tilting history of the sierra san pedro martir pluton baja california mexico from u pb 40ar 39ar and fission track geochronology the tectonothermal history of the four major phases of the sierra san pedro martir pluton and surrounding metamorphic rocks of the mesozoic peninsular ranges batholith of baja california is presented on the basis of u pb 40 ar 39 ar step heating and fission track dating in combination with al in hornblende geobarometry a previous model proposed up to 90 of east side up tilting of the pluton exposing 20 km of crustal section to account for its crescent shape asymmetrical zoning internal structure the eastward younging of k ar dates across the intrusion and eastward increase in the metamorphic grade of the country rocks from greenschist to amphibolite facies the u pb data suggest that the different phases of the pluton were emplaced sequentially from west to east between 97 0 4 1 ma and 93 8 1 1 ma all except one of the 105 40 ar 39 ar age spectra have well defined plateaus and are interpreted as cooling ages samples from the pluton give hornblende and biotite 40 ar 39 ar plateau dates and apatite fission track dates that young from west to east thus hornblende dates decrease from 95 to 91 ma biotite dates decrease from 94 to 88 ma and apatite dates decrease from 72 to 57 ma muscovite biotite and plagioclase from the same rock sample collected at the easternmost phase of the pluton yield concordant 40 ar 39 ar dates of 88 ma the exposed part of the pluton underwent rapid cooling approximately equal to 40 c ma down to 250 c in the first 10 m y after intrusion modeling of track length distribution in apatite is consistent with monotonic slow cooling from ca 80 ma to the present the data do not support a history that includes major tilting of the pluton eastward younging of 40 ar 39 ar and fission track dates may be explained by approximately equal to 15 of east side up tilting of the pluton at or after 88 ma about a north south horizontal axis furthermore the fission track data suggest that part or all of this tilting may have taken place at or after 57 ma and therefore may be a consequence of regional scale crustal extension associated with the opening of the gulf of california in neogene time such tilting is in agreement with the al in hornblende geobarometry for the hornblende biotite intrusive phase that yields pressures of 5 2 0 6 kbar an approximately equal to 15 northeast side up tilt of the crustal block containing this pluton would explain the apparent paleomagnetic inclination discrepancies with cratonic north america and militates against large scale northerly transport of baja california

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

high pressure γ camgsi2o6 does penta coordinated silicon exist in the earth s mantle in situ x ray diffraction experiments with natural fe and al bearing diopside single crystals and density functional theory dft calculations on diopside end member composition indicate the existence of a new high pressure diopside polymorph with rare penta coordinated silicon on compression diopside transforms to the phase at 50 gpa which in turn on decompression is observed to convert to the known phase below 47 gpa the new diopside polymorph constitutes another recent example of penta coordinated silicon vsi in over compressed metastable crystalline silicates suggesting that vsi may exist in the transition zone and the uppermost lower mantle in appreciable quantities not only in silicate glass and melts but also in crystalline phases contained in the coldest parts of subducted stagnant slabs vsi may have significant influences on buoyancy wave velocity anomalies deformation mechanisms chemical reactivity of silicate rocks and seismicity within the slab

high pressure phase transformation in camgsi2o6 and implications for origin of ultra deep diamond inclusions the decomposition of diopside to casio3 cubic and mgsio3 orthorhombic perovskites was demonstrated by in situ x ray diffraction measurements under simultaneous high pressure and high temperature conditions using a combination of synchrotron radiation and a multianvil apparatus at spring 8 quench experiments also confirmed the decomposition of diopside in a wide temperature range of 1000 1900 c and at pressures 23 27gpa these results strongly support the idea that some mineral inclusions with pyroxene stoichiometry discovered in natural diamonds may have been originated from the lower mantle moreover observed temperature dependence of mgsio3 content dissolved in casio3 perovskite suggests that some of the inclusions were formed at relatively low temperatures 1200 c in the deep mantle presumably related to slab subductions

high pressure phase transformations in a natural crystalline diopside and a synthetic camgsi2o6 glass abstract a phase transformation study has been carried out on a natural crystalline diopside and a synthetic camgsi 2 o 6 glass at pressures of up to 34 5 gpa and 30 gpa respectively at 1000 c in a diamond anvil cell in conjunction with a yag laser heating system on the basis of x ray diffraction data obtained from samples under in situ pressure conditions and those quenched and unloaded to the ambient conditions we found that the crystalline diopside breaks down into mg 2 sio 4 spinel sio 2 stishovite casio 3 perovskite at 17 gpa at higher pressures while the casio 3 perovskite remains stable the mg 2 sio 4 spinel and sio 2 stishovite recombine to form mgsio 3 ilmenite at 22 gpa which in turn transforms into an orthorhombic mgsio 3 perovskite at pressures above 24 gpa we have also found that the camgsi 2 o 6 glass is transformed directly into cubic ca mg sio 3 perovskite at pressures higher than 13 gpa these experimental results demonstrate that a different starting material crystalline or glassy used in a high pressure and temperature study indeed leads to different high pressure phase s thus reconciling the existing controversy on the phase transformation in diopside on the basis of the pressure volume data obtained for the cubic ca mg sio 3 perovskite it is also suggested that the cubic ca mg sio 3 perovskite although kinetically favorable to form from the camgsi 2 o 6 glass is not a thermodynamically stable phase with respect to the assemblage of the orthorhombic mgsio 3 and the cubic casio 3 perovskites at pressures between 10 and 30 gpa another interesting finding in this study is that in the case of the natural diopside stishovite is present with mmgsio 3 ilmenite casio 3 perovskite at 22 24 gpa and with mgsio 3 perovskite casio 3 perovskite at pressures above 24 gpa in view of the previous results on pure synthetic diopside in which no stishovite was observed at pressures above 22 gpa this intriguing result is most likely due to the trace amount of other cations i e fe mn na al and ti present in the sample the mechanism of precipitating the stishovite phase from ilmenite or perovskite s or both is yet unknown but the results suggest positively that mgsio 3 ilmenite or perovskite or casio 3 perovskite or both are non stoichiometric and deficient in silica

hugoniot equation of state and high pressure transformation of jadeite the hugoniot equation of state of jadeite naalsi206 was investigated with a two stage light gas gun extending the range of shock pressure up to 150 gpa from the present results coupled with previous data a high pressure phase transformation was observed in jadeite over 60 gpa moreover the hugoniot elastic limit was found to be 7 gpa fitting the theoretical hugoniot equation of state to the high pressure phase results in a bulk modulus at ambient condition of 280 10 gpa for a zero pressure density of 4 26 0 05 g cm 3 these physical properties for high pressure naalsi20 6 are consistent with the perovskite type structure at pressure above 120 gpa because its density is close to mgsio 3 and casio 3 perovskites naa1si20 6 perovskite is a candidate to be a host for sodium and aluminum near the core mantle boundary

cenozoic volcanism and two stage extension in the salton trough southern california and northern baja california pronounced differences in age and geochemistry between cenozoic volcanic rocks within the salton trough southern california and northern baja california imply their formation within two distinct tectonic settings rather than the previously proposed single phase of continental rifting the emplacement of oligocene miocene alkaline lavas overlapped with crustal extension and basin formation in the region during this time post 4 ma subalkaline magmatism related to the gulf of california east pacific rise divergent plate margin exploited crust already ruptured during miocene extension new rare earth element and sr nd isotopic data and k ar 40ar 39ar and u pb ages for basalt andesite and granophyric granite xenoliths contained within the salton buttes rhyolite domes part of the post 4 ma oceanic spreading regime indicate that they are the components of a differentiated magmatic system these data also refute the presence of crystalline basement beneath this part of the salton trough we suggest that the rupture of crystalline basement beneath the axis of the salton trough was more effective during oligocene miocene time

fault tectonics of the baja california peninsula and the opening of the sea of cortez mexico abstract a detailed field analysis of neogene and quaternary faults in baja california has enabled us to reconstruct the stress pattern and the tectonic evolution of the central and southern parts of the peninsula the deformation which is related to the opening of the gulf affects the whole peninsula but decreases from east to west most observed faults normal and or dextral strike nnw sse to wnw ese their mechanisms include both strike slip and dip slip as well as intermediate motions compressional events have occurred since late neogene times but were probably of minor quantitative importance because reverse faults are rare and small the principal fault pattern includes dextral nnw sse riedel shears and n s tension faults induced by dextral strike slip along two main nw se fault zones bordering the peninsula the gulf of california to the east which is the most important and the tosco abreojos fault to the west the resulting pattern of deformation shows that the eastern part has been a complex transform extensional zone since late miocene early pliocene times

overbank flow in compound channels with nonprismatic floodplains in this paper the authors present experimental results of overbank flow in compound channels with nonprismatic floodplains and different convergence angles the depth averaged velocity the local velocity distributions and the boundary shear stress distributions were measured along the converging flume portion for different relative depths the momentum balance is used to analyze the force acting on the flow in the main channel and for the whole cross section using the experimental data various terms in the momentum equation are also calculated the apparent shear forces on the vertical interface between the main channel and floodplains are evaluated for compound channels with nonprismatic floodplains and the results are then compared with the prismatic cases the energy balance in nonprismatic compound channels is also investigated by using the water surface elevation

u th he dating of phosphates apatite monazite and xenotime the common phosphate minerals apatite ca 5 po 4 3 f oh cl monazite ce la th po 4 and xenotime ypo 4 have found widespread use in geochronology because they incorporate u and th into their structures for example apatite usually has a few tens of ppm of both u and th while monazite and xenotime usually have hundreds of ppm to weight percent levels of these elements as a consequence these phosphates can be dated using several fundamentally different isotopic techniques elsewhere in this volume harrison et al describe phosphate dating using ingrowth of radiogenic pb the final daughter of u and th series decay and gleadow et al describe dating based on damage tracks from the spontaneous fission of 238 u the most recently developed dating technique applied to phosphates described in this chapter uses the accumulation of particles from u and th series decay u th he dating while phosphate u th pb dating is usually used to date high temperature events such as crystallization of igneous rocks and the timing of prograde metamorphism fission track and u th he dating are more commonly used to establish cooling histories through low temperatures for example in the range 110 40 c in the case of apatite dating of minerals using radiogenic he was first explored shortly after the discovery of radioactivity strutt 1908 and was investigated extensively in the 1950s and 1960s mostly on very u and th rich minerals such as zircon and titanite hurley 1952 1954 damon and kulp 1957 damon and green 1963 apatite he dating was first investigated by zeitler et al 1987 who studied the diffusion rate of he from apatite and proposed that apatite he dating might provide a useful thermochronometer recording cooling through about 100 c further studies both in the laboratory lippolt et al 1994 wolf et al 1996b warnock et al 1997 farley 2000 and in the natural setting house et al 1999 stockli et al 2000 have confirmed this idea and the technique has now been applied to a range of tectonic geologic and geomorphologic problems e g house et al 1997 1998 spotila et al 1997 farley et al 2001 stockli et al 2000 monazite and xenotime have only recently come under scrutiny for he geochronology e g pik and marty 1999 and little is yet known of their potential for routine geo or thermochronometry

asymmetrical inversely graded upstream migrating cyclic steps in marine settings late miocene early pliocene fish creek vallecito basin southern california abstract cyclic steps are ubiquitous in modern sedimentary environments yet their recognition remains sparse in the rock record here we interpret three sets of undulating backsets 1 to 3 recognized in the late miocene early pliocene latrania formation in the anza borrego desert the fish creek vallecito basin southern california usa as the first cm to dm scale outcrop record of cyclic steps based on asymmetrical cross sections upstream migration and inversely graded laminae upstream migration and asymmetrical cross sections of backsets and concomitant backset laminae are attributed to supercritical to subcritical flow transitions through weak hydraulic jumps which are composed of i thin tens of centimetres and slower reported as flow velocities of 0 45 to 1 45 m s 1 with mean value of 0 89 m s 1 subcritical represented by internal froude numbers fr of 0 67 to 0 99 with mean value of fr 0 84 turbidity currents on the stoss sides and ii thin tens of centimetres and faster reported as of 0 99 to 4 03 m s 1 with mean value of 2 24 m s 1 supercritical represented by fr of 1 84 to 3 07 with mean value of fr 2 42 turbidity flows on the lee sides the inversely graded laminae in the troughs of backsets are 2 to 5 cm thick and consist of two discrete divisions i 1 to 2 cm thick lower finer grained divisions made up of parallel laminated siltstones overlain by very fine to fine grained sandstones and ii 2 to 3 cm thick upper divisions composed of medium to coarse grained sandstones with sporadic occurrence of subrounded pebbles these inversely graded laminae are related to stratified collisional and or frictional traction carpets under conditions of high fall out rates due to the poor preservation potential of cyclic steps the rock record of cyclic steps is generally considered to be rare the present outcrop based study presents a detailed analysis of sedimentary facies growth patterns and flow dynamics of marine cyclic steps thereby contributing to better understand architectural styles and lithological properties of cyclic steps in the geological record

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

high pressure equation of state of molten anorthite and diopside new hugoniot equation of state data for molten diopside at 1773 k and molten anorthite at 1923 k are reported to 38 and 35 gpa respectively the diopside data initial density 2 61 mg m 3 are described by a straight line fit to the shock velocity particle velocity results of u s 3 30 1 44 u p km s 1 and our preferred fit to the anorthite data initial density 2 55 mg m 3 is given by u s 2 68 1 42 u p km s 1 reduction of the data to a third order birch murnaghan isentrope assuming the gruneisen ratio times the density is a constant and the mie gruneisen equation of state gives k 0s 22 4 gpa and k s 6 9 for diopside for anorthite we calculate k 0s 17 9 gpa and k s 5 3 the present data for diopside are used to calculate the diopside solidus at high pressures we expect the solidus to be shallow above 10 gpa but the lack of data on the variation of either the gruneisen parameters of the liquid and crystal or the heat capacity and thermal expansion at elevated pressures makes extrapolation of fusion curves uncertain solidus temperatures of 2400 2500 k and 2560 2705 k for diopside are calculated at 10 and 20 gpa respectively the new data are combined with those of rigden et al 1988 for the di 0 64 a 0 36 eutectic composition to examine the degree to which such liquids mix ideally with respect to volume up to 25 gpa for the eutectic composition at 1400 c we calculate the volumes of the an and di mix nearly ideally to 25 gpa we find that the ratio of the partial molar volumes of the oxides in silicate melts to that of the crystal oxides at 1673 k and 1 atm is 1 0 0 1 for a wide range of oxide components for the low pressure tetrahedrally coordinated oxides e g sio 2 al 2o 3 fe 2o 3 the ratio is 1 3 with respect to oxides such as stishovite corundum and hematite in which the cations are octahedrally coordinated by oxygens if changes in coordination of al and si from tetrahedral at low pressures to octahedral at high pressures occur in melts they do so gradually over an interval of 40 gpa although 1 atm bulk moduli for a wide compositional range of silicate melts are similar the differences in integrated compression to mixed oxide like high pressure configurations are reflected mainly by variations in k t k t is found to vary inversely with fraction of network forming initially tetrahedrally coordinated cations e g al 3 si 4 thus k t which may be uncertain by 1 5 is estimated to vary from 7 for molten anorthite enstatite and diopside to 8 for molten ferrosilite to 10 for molten forsterite and to 11 for molten fayalite

turbulent open channel flows with variable depth across the channel the flow of water in straight open channels with prismatic complex cross sections is considered lateral distributions of depth mean velocity and boundary shear stress are derived theoretically for channels of any shape provided that the boundary geometry can be discretized into linear elements the analytical model includes the effects of bed generated turbulence lateral shear turbulence and secondary flows experimental data from the science and engineering research council serc flood channel facility are used to illustrate the relative importance of these three effects on internal shear stresses new experimental evidence concerning the spatial distribution of reynolds stresses yx and zx is presented for the particular case of compound or two stage channels in such channels the vertical distributions of zx are shown to be highly nonlinear in the regions of strongest lateral shear and the depth averaged values of yx are shown to be significantly different from the depth mean apparent shear stresses the importance of secondary flows in the lateral shear layer region is therefore established the influence of both reynolds stresses and secondary flows on eddy viscosity values is quantified a numerical study is undertaken of the lateral distributions of local friction factor and dimensionless eddy viscosity the results of this study are then used in the analytical model to reproduce lateral distributions of depth mean velocity and boundary shear stress in a two stage channel the work will be of interest to engineers engaged in flood channel hydraulics and overbank flow in particular

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

age and stratigraphic relationships of pre and syn rift volcanic deposits in the northern puertecitos volcanic province baja california mexico geologic mapping of volcanic strata of the northern puertecitos volcanic province pvp in northeastern baja california mexico performed in conjunction with 40 ar 39 ar analysis and petrochemical study documents the miocene geologic history of a well preserved volcanic succession within the northern sierra santa isabel and its relationship to the evolving pacific north america plate boundary subduction related volcanic deposits well exposed in profile along the northern margin of the pvp in the informally named santa isabel wash region span pre 17 to 15 ma minor rift related volcanism occurred at 12 5 and 9 ma prior to voluminous pvp forming volcanism at 6 6 5 ma isochron ages typically exhibit precision 1 for plagioclase of 2 7 and for anorthoclase and sanidine of 2 5 and replicate analysis of an internal anorthoclase standard indicate 1 2 reproducibility within a given irradiation and 2 5 for samples irradiated separately improved local correlations made possible by the rich stratigraphic section preserved in santa isabel wash help constrain the relationships of several widespread pyroclastic flow deposits in northeastern baja california these correlations are important for both paleomagnetic studies within the region and for establishing geologic ties across the gulf of california the combined mapping and age results imply that most extensional deformation in the study area is post 6 ma although some earlier faulting and the development of the pre 6 ma matomi accommodation zone are also documented results support a transitional plate boundary model which implies that much of the pacific north america relative plate motion north of delfin basin i e the northernmost gulf of california is accommodated on n to nnw striking faults developed during late miocene ene directed extension the model predicts a zone of divergence east of the pvp which provides a structural mechanism for the positions and jumps of nearby gulf of california spreading centers upper and lower tiburon and delfin basins since 6 ma and relates major pulses of pvp volcanism at 6 and 3 ma to these offshore spreading center adjustments results also imply that most extensional deformation in santa isabel wash is the result of incorporation of the pvp into the gulf extensional province 2 3 ma due to northwestward propagation of the guaymas fracture zone rotational deformation north of the pvp may have begun contemporaneously with this adjustment along the gulf extensional province rift margin

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

an age constraint on gulf of california rifting from the santa rosalia basin baja california sur mexico marine rocks of the santa rosalia basin baja california sur were sampled in a pilot study to determine their suitability for magnetostratigraphy and geochronology with the goal of providing an age constraint on gulf of california rifting progressive demagnetization of samples from the boleo formation the earliest marine sequence overlying the deeply eroded basement reveals a high coercivity characteristic remanent magnetization chrm in addition to a low coercivity overprint the chrm appears to be a primary magnetic remanence with stratigraphically bound normal and reversed polarity directions a single 40 ar 39 ar isotopic age of 6 76 0 90 ma 2 was obtained for the cinta colorada a tephra deposit of reversed paleomagnetic polarity within the boleo formation the age of the cinta colorada is refined by calculating isotopic age probabilities for each of the reversed polarity intervals of the geomagnetic polarity time scale gpts in the 2 range 5 86 7 66 ma the interval with the highest probability is 6 57 6 94 ma in conjunction with the isotopic age preliminary magnetostratigraphy of the boleo formation is correlated with the gpts in order to further delineate the onset of marine sedimentation the most likely correlation yields an age of 6 93 7 09 ma gpts subchron c3bn for the base of the boleo formation and 6 14 6 27 ma for the top this correlation indicating an average sedimentation rate of 28 4 cm k y could be significantly altered if a more thorough magnetostratigraphy proved the existence of additional polarity intervals in the boleo formation however even if the isotopic age of the cinta colorada is used as the only age constraint the result is consistent with data from the northern gulf of california and shows that rifting started much earlier than the ca 3 6 ma commencement of sea floor spreading at the mouth of the gulf of california the deposition of the boleo formation was probably related to an early phase of gulf rifting caused by a change in pacific north american plate motions