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

paleomagnetic evidence for the miocene counter clockwise rotation of northeast japan rifting process of the japan arc paleomagnetic results have been obtained from more than 700 oriented samples in igneous rocks of cretaceous to miocene age from northeast japan the remanent magnetizations of welded tuffs with age between 32 ma and 21 ma old from 17 widely distributed sampling sites in northeast japan are fairly well grouped with a mean ofd 41 2 i 56 5 95 7 2 the remanent magnetization of welded tuffs of 14 ma and 11 ma from 4 sites shows a northerly direction d 11 7 i 60 5 95 15 3 these data imply that northeast japan was apparently subjected to a counter clockwise rotation through 47 14 about a vertical axis on the representative point of northeast japan 141 e 39 n during the past 21 ma comparing with the period of the clockwise rotation of southwest japan the differential rotation between northeast japan and southwest japan is concluded to have occurred concurrently during the period between 21 ma and 12 ma a rifting model of the japan arc is proposed northeast japan rotated counter clockwise through 50 about a northern pivot at 146 e 44 n while southwest japan rotated clockwise through 54 about a southern pivot at 129 e 34 n the differential rotation about different rotation pivots gives rise to the opening of a diamond shaped back arc basin the japan sea

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

seismic activity and stress tensor inversion at las tres virgenes volcanic and geothermal field mexico abstract we analyze local earthquakes occurring between 2003 and 2012 at the las tres virgenes volcanic and geothermal field tvvgf to establish their temporal and spatial distribution and relationships with local and regional fault systems water injection acid stimulation and steam production tests we obtained focal mechanisms and inverted data for the stress tensor to understand the local and regional stress fields we analyzed 423 local earthquakes with magnitudes between 0 1 and 2 9 mc and hypocentral depths from 0 2 to 7 4 km b s l the cutoff depth at 7 4 km possibly delineates the brittle ductile transition zone we identified seven swarms from 1 to 7 swarms 1 december 2009 2 may 2010 3 june july 2010 and 7 december 2012 are strongly correlated with injection processes whereas swarms 5 april 2012 and 6 september 2012 are correlated with local tectonic faults stress inversion showed nw se e w and ne sw extensional orientations s hmin in agreement with the local tectonic stress field while ne sw compressional orientations s hmax are correlated with the regional tectonic stress field

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

tree ring dating of pre 1980 volcanic flowage deposits at mount st helens washington tree ring dating ring width pattern matching was used to determine emplacement dates for eight subfossil tree bearing pre 1980 pyroclastic flow and lahar deposits in stream valleys draining mount st helens washington limiting dates were also established for nine other pre 1980 flowage deposits from the ages of trees rooted on or near them or from exhumed trees the 17 new dates improve understanding of the chronology of events during and after mount st helens9s kalama eruptive period which extended from a d 1479 to the mid 1700s we assign bracketing dates to the three phases of the kalama eruptive period 1479 1510 to the early kalama during which mount st helens erupted explosive dacitic tephras pyroclastic flows and one or more domes 1489 1566 to the middle kalama during which mount st helens produced andesitic tephras pyroclastic flows and lava flows and 1489 1750 to the late kalama during which mount st helens again erupted dacite overlap of the brackets reflects the limits of our dating resolution the revised chronology confirms that little time perhaps 50 yr elapsed between the end of mount st helens9s kalama eruptive period and the start of the succeeding goat rocks eruptive period a d 1800 1857 eruptions apparently occurred sporadically during the entire 1479 1857 interval the record also indicates that a large pyroclastic flow occurred 7 10 yr after the explosive eruptions that began the kalama eruptive period the dates further imply that extrusion of the pre 1980 summit dome continued intermittently for 100 yr and limit the ages of two tephras these findings demonstrate the usefulness of relict tree bearing deposits to volcanic history studies

subduction zones an introduction to comparative subductology abstract the subduction model for trench arc back arc systems was deduced more or less as a logical consequence of sea floor spreading many features in these systems such as thrust type inter plate earthquakes and deep earthquakes along wadati benioff zones are readily explained by the subduction model but other features such as extensional spreading and high heat flow in back arc regions and arc volcanism are difficult to explain an approach to solve these problems may be to recognize the existence of the two basically different modes of subduction one chilean type causing a compressional stress regime in the arc and back arc regions the other mariana type causing a tensional stress regime implications of the two types of the mode of subduction for some tectonic problems such as vertical movement and sediment accretion at trenches are discussed with special reference to the results of recent dsdp active margin drilling in discussing the possible causes of the two modes it is suggested that the large scale thermal and stress regimes in the back arc areas may not be the direct products of simple subduction but of some additional factors involving much greater energy such as mantle flow associated with the motion of major plates finally the possible great importance of various types of collision accretion and erosion at subduction zones is emphasized

the influence of volcanism on fluvial depositional systems in a cenozoic strike slip basin denali fault system yukon territory canada abstract the depositional history of the eocene oligocene burwash strike slip basin is characterized by a transition from non volcanic clastic sedimentation of the amphitheatre formation to deposition of lavas and volcaniclastic rocks of the overlying lower wrangell volcanic sequence the lowermost wrangell volcanic deposits within the burwash basin consist mostly of 110 m of trachybasalt basaltic trachyandesite and trachyte lava flows which directly overlie about 440 m of stream dominated alluvial fan fan delta and lacustrine deposits of the amphitheatre formation overlying the lava flows are about 50 m of interbedded volcaniclastic fluvial deposits pumice rich andesitic pyroclastic fallout deposits and lithic rich trachyandesitic pyroclastic flow deposits although volcanism is known to occur within strike slip basins the role of volcanism in strike slip basin evolution and the effects of volcanism on sedimentation in strike slip basins are largely unknown the depositional history of the sedimentary rocks amphitheatre formation and the volcanic rocks wrangell volcanic sequence in the burwash basin can be divided into four progressive stages 1 progradational stream dominated alluvial fan deposition 2 lacustrine deposition 3 renewed basin margin faulting proximal alluvial fan deposition and initial volcanic vent formation and 4 intrabasinal volcanism resulting in lava flow emplacement pyroclastic eruptions and fluvial volcaniclastic sedimentation this study focuses on the complex interplay between volcanism and fluvial sedimentation that can be demons rated during stages 3 and 4 and on the overall reorganization of depositional systems in this strike slip basin during the transition from non volcanic clastic sedimentation to lava flows and volcaniclastic sedimentation paleocurrent data collected from the burwash basin document major reorganization of paleodrainage during the clastic to volcaniclastic transition prior to volcanism fluvial systems flowed westward toward and along the basin axis whereas after the onset of volcanism fluvial systems drained northeastward outward from the basin center in addition conglomerate compositions in the basin change at the clastic to volcaniclastic transition from extrabasinally derived basement clasts eroded from uplifted source areas along the strike slip basin margin to intrabasinally derived volcanic clasts fluvial depositional style was also strongly influenced by contemporaneous explosive volcanism during the late stage of burwash basin evolution fluvial aggradation which occurred in the form of sand dominated hyperconcentrated flood flows and braided streams was contemporaneous with episodes of pyroclastic activity these syneruption fluvial deposits are sheet form and contain abundant juvenile detritus vitric grains and pumice euhedral to subhedral and angular crystal fragments of plagioclase and hornblende and a small percentage of basaltic lithic fragments fluvial degradation marked by the incision of steep valleys occurred during periods between eruptions the inter eruption fluvial deposits are characterized by lenticular beds of clast supported conglomerate and trough rossbedded sandstone and are composed predominantly of basaltic lithic grains with small amounts of crystal and vitric fragments subsequent eruptions generated pyroclastic flows which ponded in the fluvial valleys these pyroclastic flow deposits exhibit crystal rich bases lithic rich lapilli sized middle zones with out sized lithic blocks and gas escape structures upper pumice rich zones and interbedded stratified surge deposits our results show that the onset of volcanism in the burwash basin produced a major reorganization of fluvial drainage systems and sediment source areas in addition changes in fluvial depositional style were closely linked to alternating periods of explosive volcanic activity and quiescence during the late stages of strike slip basin evolution the excellent exposures in the burwash basin provide an apt opportunity to examine the role of volcanism in the depositional history of an intracontinental strike slip basin

pyroclastic surges of the pleistocene monte guardia sequence lipari island italy depositional processes the 20 16 ka monte guardia sequence of lipari island southern italy is a complex succession of silicic pyroclastic surge deposits produced in part by hydromagmatic explosions near sea level most surges were directed to the east north east and north of the vent and climbed the 12 southern slopes of monte sant angelo in the central part of the island a series of thin distinctive key bed sets containing oxidized ash and accretionary lapilli allow a detailed correlation of sections and the lateral tracing of deposits of single pyroclastic surges across the island facies analysis reveals that the proximal to distal facies changes are different from those suggested by a previous study based on a statistical approach to lateral facies distribution single dry surge deposits evolve downcurrent from 1 beds of disorganized medium to coarse grained lapilli containing scattered blocks to 2 bipartite disorganized stratified beds of fine to coarse grained lapilli with ash matrix to 3 dunes formed of coarse grained ash to medium grained lapilli to 4 planar beds of fine grained lapilli this facies sequence is similar to published models for some korean surge deposits and records decelerating surges which experienced a downflow decrease in turbulence particle concentration and suspended load fall out rate and an increase in traction processes as the monte guardia surges climbed the opposing slopes of monte sant angelo they bifurcated into eastern and western tongues which experienced rapid deceleration leading to a rapid downcurrent thinning and fining of the surge deposits two fluid dynamical approaches suggest that monte guardia surges travelled at speeds of more than 75 85 m s 1 before climbing monte sant angelo flows with this vigour and distribution are capable of destroying animal and plant populations on lipari

dynamics and implications of slab detachment due to ridge trench collision 1 the approach of a buoyant spreading ridge to a subduction zone may lead to detachment of a subducted slab previous work has called upon the detachment process as an explanation for observed ridge abandonment and slab window related magmatism e g in baja california but such a scenario has not been tested using fully dynamic numerical models we use dynamic two dimensional models including a non newtonian rheology to study the approach of a trench parallel spreading ridge to a subduction zone we find that before the ridge approaches within 100 km of the trench detachment of the subducted slab occurs due to the combined effects of increased buoyancy and reduced strength of the lithosphere with ridge proximity in models exploring effects of subducted slab length distance of the ridge from the trench shear zone strength and lithospheric yield strength we find the following 1 a decrease in subduction velocity as the ridge approaches the trench 2 a shrinking surface plate that maintains a uniform plate like subduction velocity 3 ridge abandonment distances 100 275 km from the trench and 4 slab gap distances 195 285 km from the trench these results are consistent with observations in baja california where detachment of the cocos slab may explain abandonment of observed segments of the east pacific rise 50 200 km outboard of the trench and the presence of nonarc related magmatism located 100 250 km inboard of the trench ridge subduction does not occur within any of these two dimensional cases of ridge trench collision indicating that 3 d geometric effects may be responsible for ridge subduction

late pliocene to early quaternary extensional detachment in the la pazâel cabo area baja california sur mexico implications on the opening of the gulf of california and the mechanics of oblique rifting terra nova 22 64 69 2010 abstract we demonstrate that pliocene to early quaternary sedimentary formations in baja california sur mexico were deposited syn tectonically over a major detachment associated with the exhumation of mesozoic crust the detachment dips to the ene and is associated with e w stretching this large extensional structure strikes almost parallel to the general trend of the gulf of california and extension is oblique to the east pacific seafloor spreading direction crustal scale stretching in this area was still active after the beginning of seafloor spreading c 3 6 ma ago the detachment is capped by late pleistocene holocene alluvial sediments the deposition of which seems to be partly syn tectonic and controlled by minor stretching subparallel to the present day north american pacific kinematic vector we discuss the implications of our observations on strain partitioning during opening of the california gulf as well as on the structure of the gulf of california margin

active deformation and shallow structure of the wagner consag and delfin basins northern gulf of california mexico oblique rifting began synchronously along the length of the gulf of california at 6 ma yet there is no evidence for the existence of oceanic crust or a spreading transform fault system in the northern gulf instead multichannel seismic data show a broad shallow depression 70 200 km marked by active distributed deformation and six 10 km wide segmented basins lacking well defined transform faults we present detailed images of faulting and magmatism based on the high resolution and quality of these data the northern gulf crust contains a dense up to 18 faults in 5 km complex network of mainly oblique normal faults with small offsets dips of 60 80 and strikes of n n30 e faults with seafloor offsets of tens of meters bound the lower and two upper delfin basins these subparallel basins developed along splays from a transtensional zone at the nw end of the ballenas transform fault twelve volcanic knolls were identified and are associated with the strands or horsetails from this zone a structural connection between the two upper delfin basins is evident in the switching of the center of extension along axis sonobuoy refraction data suggest that the basement consists of mixed igneous sedimentary material atypical of mid ocean ridges on the basis of the near surface manifestations of active faulting and magmatism seafloor spreading will likely first occur in the lower delfin basin we suggest the transition to seafloor spreading is delayed by the lack of strain partitioned and focused deformation as a consequence of shear in a broad zone beneath a thick sediment cover

origin and early evolution of lahar channel at mabinit mayon volcano philippines mabinit channel 5 km long 15 to 70 m wide and 2 to 22 m deep was formed by lahars on the southeastern slope of mayon volcano during its 1984 eruption in 1985 the channel was drastically modified by a lahar triggered by a typhoon sediment budget calculations from surveys conducted in 1985 and 1986 corroborate inferences from other volcanoes that lahars can grow significantly in volume by eroding their channels the 1984 eruption produced a deep summit ravine and at its base a fan of avalanche and pyroclastic flow deposits 2 km2 in area and 40 x 106 m3 in volume lahars initiated in the ravine by heavy rains from eruption updrafts carved out 5 km long mabinit channel along the west margin of the pyroclastic avalanche field a position that could not have been predicted from pre eruption topography by eroding the channel the lahars generated more than half of their own solid contents below 240 m where the volcano slope decreases from 5 to 3 the first eruption debris flows were unconfined and spread out over a 512 000 m2 area depositing a layer locally more than 4 m thick with a volume of 1 25 x 106 m3 the thin margins of these unconfined flows were non erosional however as they stopped their thick central portions continued flowing extending the new channel downslope through the depositional field subsequent eruption lahars modified the channel and overflowed at its bends a moderately intense typhoon in october 1985 triggered a single 9 hr lahar event that widened the channel by an average of 25 m and caused as much as 66 m of lateral bank erosion debris flows overtopping the channel at four sites coalesced to cover a 2 km long 200 000 m2 area with bouldery sediment in wider channel stretches debris flow margins left prominent paired levees while their central portions continued moving down channel as rigid plugs the lahar plugged the lower 0 5 km stretch of the channel and replaced it by eroding a new channel of comparable size this demonstrated how easily lahars can change the courses of channels on the upwardly convex debris apron of a stratovolcano with serious consequences for farmland and communities

relationship between tectonics volcanism sedimentation and basin development late cenozoic central part of northern honshu japan abstract a pronounced interplay between tectonics volcanism and sedimentation can be established in the volcanic arc region of the central part of northern honshu during the early miocene to quaternary interval four stages are recognized with the type of volcanism and sedimentation varying according to the tectonic stress regime 1 1 rifting stage 22 15 ma ns trending half grabens were formed under an extensional stress regime related to the rifting of the sea of japan basins were rapidly filled by intermediate to felsic volcanic products which are often associated with subaerial fluvial and lacustrine sediments 2 2 back arc basin opening stage 15 13 ma with more advanced stagess of rifting subsidence from crustal stretching resulted in marked marine incursion and an overall transgressive succession developed during this stage the back arc side of northern honshu subsided rapidly to middle bathyal environments at the same time associated with the opening of the sea of japan large volumes of regionally extensive mafic lavas and associated hyaloclastites were erupted within submarine grabens in contrast in the present backbone ranges felsic volcanism occurred within bathyal to shallow marine environments igneous rock types in this stage display a bimodal mafic felsic chemistry 3 3 transitional stage 13 2 4 ma a thermal subsidence substage 13 8 ma an intermediate stress regime developed which included phases of both weak extension and compression consequently grabens along the sea of japan ceased to form later lithospheric cooling eventually led to thermal subsidence of the back arc region and hemipelagic sediments mantled the previously active mafic volcanic apron volcanism at this time was mainly restricted to a narrow zone along the present backbone ranges where felsic to intermediate volcanic products accumulated within shallow marine environments b incipient compressional substage 8 2 4 ma hemipelagic sedimentation continued on the back arc side of northern honshu along the present backbone ranges the intermediate stress regime continued with formation of many large caldera structures associated with subaerial felsic volcanism 4 4 shortening deformation stage 2 4 ma present the change of the absolute direction of motion of the pacific plate at the early to middle pliocene changed the regional stress regime of the volcanic arc from intermediate to compression movement of back arc thrusts along the sea of japan coast began 2 3 ma and led to uplift of the volcanic arc continued compression in the volcanic arc produced narrow uplifted zones with associated basins bounded by reverse faults these basins were filled rapidly by terrestrial sediments containing upward coarsening sequences under this highly compressional tectonic stress regime volcanism became intermediate mafic in composition with large stratovolcanoes becoming prominent

paleocurrent and fabric analyses of fluvial conglomerates of the paleogene noda group northeast japan abstract clast supported fluvial conglomerates of the paleogene noda group northeast japan consist of two contrasting formations the lower minato formation comprising dominantly acidic volcanic gravels and the conformably overlying kuki formation of mesozoic basement gravels crudely horizontally stratified or massive sheets of conglomerates facies gm comprise most of the sediments of both the formations and show no abrupt lateral changes of facies debris flow deposits are absent from the sediments of the group paleocurrent analysis from gravel imbrication of facies gm suggests that sediments of the minato formation were deposited by eastward paleoflow and had multiple sources in the western volcanic range such characteristics of the formation seem to show contrasting sedimentological aspects when compared with a typical alluvial fan or a fan delta sediments of the lower part of the group were formed as a coalescing braid delta elsewhere denudation of lower mesozoic basement rocks in the northern source area provided sources for the uppermost part of the group kuki formation fabric and structure of the deposits give some clues to understand the complexities of small scale fluvial topographies symmetrical or asymmetrical concave up gravel alignments of the conglomerate facies gt suggest a small scale channel dissecting obliquely into a broad longitudinal bar of facies gm elsewhere dip orientations of imbricated gravels of facies gm coincide well with the maximum dip direction of cross bedding within the overlying sand bar facies sp however the facies change from gm to sp is remarkable features of gravel imbrication provide useful information to understand bounding surface problems

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

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

opening of the sea of japan back arc basin by asthenospheric injection abstract the position of a volcanic front on an arc is governed by dehydration reactions of amphibole chlorite in down dragged hydrated peridotite at the base of the mantle wedge as these reactions are essentially pressure dependent ca 3 5 gpa the paleo position of a volcanic front offers an estimate for the depth of the subducted oceanic lithosphere beneath it this paper evaluates volcanism in the northeast japan arc since the oligocene and discusses the mass transportation in the mantle wedge during the episode of back arc extension in the sea of japan with reference to the migration of the volcanic front the volcanic front during the oligocene ca 30 ma was located along the western coast of the present northeast japan are and migrated about 200 km trenchwards at 23 ma before the opening of the sea of japan back arc basin it follows that the angle of subduction became steeper during the period of 30 23 ma triggered by the injection of the asthenosphere into the mantle wedge the back arc extension of the sea of japan between 21 and 14 ma is a manifestation of the dynamic process of the lithosphere due to the asthenospheric injection the temperature of the injecting asthenospheric mantle was higher than that of the surrounding upper mantle or lithosphere this caused the characteristic volcanism at 23 22 ma 1 high magnesian andesite volcanism took place in the near trench region 2 across arc compositional variation was not observed 3 the volume of volcanic materials increases towards the back arc side of volcanic arc the volcanic front has migrated over a short distance inwards less than 100 km since 15 ma possible as a result of cooling in the mantle wedge because both amphibole and chlorite break down at slightly higher pressures at lower temperatures the upper mantle materials which injected into the mantle wedge and caused back arc spreading are identical to those of the mid oceanic ridge basalt source in sr nd isotopic compositions because volcanic rocks in the back arc region of the northeast japan arc show secular variation in isotopic compositions from enriched to depleted isotopic characteristics during the last 30 ma

basin evolution within the kitakami massif northeast japan relationship between sedimentation tectonics and volcanism in an incipient neogene continental back arc basin abstract the ichinohe basin in the western margin of the kitakami massif northeast japan consists of the early to middle miocene yotsuyaku and keiseitoge formations and represents the incipient stage of neogene back arc sedimentation that extends toward the north and northwest megaclast breccias derived from mesozoic basement rocks of the kitakami massif were transported from southern areas as debris flows the debris flow deposits of the yotsuyaku formation grade northwards along the basin axis into clast supported braided stream deposits where floodplains developed in the center of the basin voluminous volcanic eruptives of the keiseitoge formation were derived from eastern and southeastern volcanic ranges and formed transversal distributaries the deposition of both clastic and pyroclastic sediments suggests that volcanism and tectonic uplift took place simultaneously during basin evolution parallel laminated fine grained sediments intercalated with lignite beds suggest lacustrine deposition and large cross beds of volcanic breccias indicate gilbert type progradation into the lakes a number of small lakes in the center of the basin was probably formed as the result of damming tributary mouths by the main drainage from eastern and southeastern volcanoes or the reciprocal blocking by lahars and non volcanic debris flows derived from the southern uplift damming episodes produced by thick lava flows were not recognized in this study the evolution of the n s trending longitudinal ichinohe basin indicates that e w extensional tectonism related to the opening of the sea of japan affected not only the green tuff region to the west of the studied basin but also even the mesozoic basement during the miocene although the mingling of volcanic and basement derived deposits indicates the complexities of sedimentary facies and provenances our view of the stratigraphic architecture has relevance to the interpretation of the evolution of other extensional basin systems

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

thick deltaic sedimentation and detachment faulting delay the onset of continental rupture in the northern gulf of california analysis of seismic reflection profiles 1 the transition from distributed continental extension to the rupture of continental lithosphere is imaged in the northern gulf of california across the obliquely conjugate tiburon upper delfin basin segment structural mapping on a 5 20 km grid of seismic reflection lines of petroleos mexicanos demonstrates that 1000 extension is accommodated on a series of nne striking listric normal faults that merge at depth into a detachment fault the detachment juxtaposes a late neogene marine sequence over thinned continental crust and contains an intrabasinal divide due to footwall uplift two northwest striking dextral oblique faults bound both ends of the detachment and shear the continental crust parallel to the tectonic transport a regional unconformity in the upper 0 5 s two way travel time and crest erosion of rollover anticlines above the detachment indicates inversion and footwall uplift during the lithospheric rupture in the upper delfin and lower delfin basins the maximum length of new crust in both delfin basins is less than 40 km based on the lack of an acoustic basement and the absence of a lower sedimentary sequence beneath a wedge shaped upper sequence that reaches 5 km in thickness a fundamental difference exists between the tiburon delfin segment and the guaymas segment to the south in terms of presence of low angle normal faults and amount of new oceanic lithosphere which we attribute to thermal insulation diffuse upper plate extension and slip on low angle normal faults engendered by a thick sedimentary lid

mode of extension and rifting history of upper tiburon and upper delfin basins northern gulf of california 1 the crustal structure of the northern gulf of california transtensional margin has been investigated by a 280 km long nw se profile including deep multichannel seismic reflection and densely sampled refraction wide angle reflection seismic information combined with gravity modeling the seismic and gravity modeling constrains two thinned crustal areas corresponding to the upper delfin and the upper tiburon basins on both sides of the profile toward the baja california peninsula and the mexico mainland a progressive thickening of the continental crust is observed our results indicate that the crustal thickness is 19 km below the coastline and it decreases to 14 and 17 km below the upper delfin and upper tiburon basins respectively in the area between both basins the crust thickens to 19 5 km there are significant lateral thickness variations for the different levels of the crust the interpreted structure is consistent with the existence of an aborted rift below the upper tiburon basin prominent dipping reflections in the multichannel data under upper tiburon basin and the ridge between upper tiburon and upper delfin basins can be explained as a mylonite like zone related to a detachment fault this interpretation suggests that the structural evolution of upper tiburon basin could be controlled by a major fault that cuts through the upper crust and merges into a zone of subhorizontal reflections in the lower crust the mode and locus of extension have evolved from a core complex in upper tiburon to a narrow rift mode in upper delfin basin

evolution of a plio pleistocene volcanogenic alluvial fan the puye formation jemez mountains new mexico the plio pleistocene puye formation north central new mexico contains 15 km 3 of volcaniclastic alluvial sediments deposited in response to rift margin volcanism associated with the later evolution of the tschicoma volcanic center northeastern jemez mountains stratigraphic and sedimentologic evidence indicates that aggradation of puye deposits was nearly continuous and contemporaneous with graben development and establishment of the rio grande as the through flowing axial drainage system in this region much of the puye sediments debouched into an adjacent subsiding depocenter resulting in near complete preservation of this wedge shaped volcanogenic fan primary pyroclastic and reworked facies in the puye formation exhibit distinctive 5 to 30 m scale cyclicity directly related to volcanic activity in the northeastern jemez highlands reworked pyroclastic and conglomeratic sequences deposited by high energy braided streams sediment charged sheet floods sediment gravity flows and in shallow lakes reflect markedly increased sediment loads during and following explosive eruptions individual sequences change markedly in character with increased distance from the source area analysis of these abrupt lateral and vertical variations reveals that puye sedimentation was controlled by the influences of semiarid climate and syndepositional tectonism although the associations that most characterize the fan resulted largely from explosive volcanism cessation of fan growth is attributed to waning sediment supply as a result of volcanic quiescence and to the onset of basinwide pedimentation associated with downcutting of the ancestral rio grande the puye fan therefore provides a detailed record of rift basin alluvial sedimentation complete with closely spaced tephrostratigraphic control and it records the growth and partial denudation of high standing northeastern jemez mountains volcanoes

sedimentation and crustal recycling along an active oblique rift margin salton trough and northern gulf of california transtensional basins embedded in the san andreas fault system of southern california united states and northwestern mexico are fi lled with sediment derived from the colorado river which drains a large area of the western u s interior the sediment is rapidly buried heated and mingled with intrusions in the deep basins to form a new generation of recycled crust along the active plate boundary using a range of values for total basin depth relative volume of mantle derived intrusions and composition of early rift deposits the volume of colorado river derived sediment in the basins is bracketed between 2 2 and 3 4 10 5 km 3 similar to the volume of rock that likely was eroded from the colorado river catchment over the past 5 6 m y the volumetric rate of crustal growth by sedimentation is 80 130 km 3 m y km comparable to growth rates in subductionrelated island arcs and slow seafl oor spreading centers sedimentary and basinal processes thus play a major role in crustal evolution and recycling in this setting and may be important at other rifted margins where a large river system is captured following tectonic collapse of a prerift orogenic highland

slip inversion along inner fore arc faults eastern tohoku japan funding was provided by a grant from the national science foundation tectonics program grant ear 0809939 to d m f and e k geologic society of america graduate research grants and the p d krynine memorial fund the authors thank gaku kimura kyoko tonegawa hiroko watanabe jun kameda and asuka yamaguchi for scientific and logistical support and kristin morell for comments on early versions of the manuscript we also thank yuzuru yamamoto and kohtaro ujiie for their detailed reviews and suggestions for improvement to the manuscript the authors acknowledge the use of the move software suite granted by midland valley s academic software initiative geologic structural stratigraphic and chronologic data used herein are accessible in manuscript figures and in the citations therein input geologic data for trishear kinematic modeling can be accessed in table 1 and in the supporting information ear 0809939 national science foundation tectonics program grant geologic society of america graduate research grants p d krynine memorial fund

farallon slab detachment and deformation of the magdalena shelf southern baja california 1 subduction of the farallon plate beneath northwestern mexico stalled by 12 ma when the pacific farallon spreading ridge approached the subduction zone coupling between remnant slab and the overriding north american plate played an important role in the capture of the baja california bc microplate by the pacific plate active source seismic reflection and wide angle seismic refraction profiles across southwestern bc 24 5 n are used to image the extent of remnant slab and study its impact on the overriding plate we infer that the hot buoyant slab detached 40 km landward of the fossil trench isostatic rebound following slab detachment uplifted the margin and exposed the magdalena shelf to wave base erosion subsequent cooling subsidence and transtensional opening along the shelf starting 8 ma starved the fossil trench of terrigenous sediment input slab detachment and the resultant rebound of the margin provide a mechanism for rapid uplift and exhumation of forearc subduction complexes

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

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

orogeny and relative plate motions example of the japanese islands abstract the paleogeography of the eastern margin of asia near japan for the past 250 ma has been reconstructed on the basis of the relative motions between the oceanic plates and the eurasian plate engebretson et al 1982 and on land geology the relationships between the plate kinematics and orogeny are as follows calc alkaline magmatism is related to the presence of subduction when a plate boundary is a transform fault there is no volcanism very slow convergence rates do not give rise to magmatism migration of rft type triple junctions or ridge subduction produces forearc volcanism near the trench and also a wider volcanic zone uplift of low p t type regional metamorphic belts is related to collision episodes the uplift of the high p t blueschist belts on the contrary is not contemporaneous with them they appear to be related to the start of the subduction of a young plate after the migration of an rft type triple junction because the formation of a voluminous accretionary complex is contemporaneous with this and thrusting of this young accretionary complex beneath the uplifted metamorphic belt is often observed the underplating of the young accretionary complex beneath the older accretionary complex would be the cause of the uplift of the high p t type metamorphic belt the subduction of a young plate would thus be an important factor causing these orogenic events highly oblique convergence also results in formation of a serpentinite melange zone with fragments of blueschists presumably along a mechanical plate boundary in the forearc

distribution of quaternary deformation in the central main ethiopian rift east africa 1 the main ethiopian rift mer is a narrow continental rift characterized by an along axis variation in rift evolution with early stages in the south evolving to incipient breakup in the north although distribution and style of quaternary volcanotectonic deformation is well known in the northern rift sector knowledge of these characteristics is comparatively less constrained southward in this paper we present the results of a field structural study carried out to better constrain the time space distribution of faulting in the central sector of the mer central mer the new field structural data coupled with new 14c radiometric dating of faulted rocks suggest a localization of faulting at both rift margins of the central mer where radiometric dating of faulted material has allowed establishing a late pleistocene holocene activity of border faults conversely in rift faulting wonji fault belt wfb is subordinate highlighting a major difference with the northern sector of the mer where deformation is essentially accommodated in the axial zone this is consistent with an along axis variation in rift evolution showing the central mer less evolved than the northern rift sector inversion of cumulative fault slip data reveals a variation in the extension direction between the rift margins n105 110 e and the rift floor n90 95 e which accords well with the current nubia somalia plate kinematics the variation in extension direction across the rift could manifest a slip partitioning between the boundary faults and in rift wfb faults

south kitakami region an allochthonous terrane in japan the pre cretaceous geology of the japanese islands may be interpreted as an accretion complex in which many terranes of several origins are juxtaposed and a number of allochthonous blocks are involved the south kitakami and abukuma regions of northeast japan and probably also silurian and devonian blocks in the kurosegawa fault zone and the border fault zone of the hida metamorphic terrane are the allochthonous blocks the stratigraphic sequence and paleobiofacies of the regions are similar to each other and their lithologic characters are of continental affinity but however are quite different from those of the other terranes having oceanic characters the south kitakami and abukuma regions have been fragments of a lost continent less remotely situated to gondwana during the middle paleozoic and may have drifted and incorporated with the other part of the japanese islands before late mesozoic to early tertiary igneous activity

lahar facies of the mount dutton formation oligocene miocene in the marysvale volcanic field southwestern utah lahar facies of the mount dutton formation oligocene miocene marysvale volcanic field utah differ in clast size gradation presence or absence of a self supporting clast framework and to a lesser extent thickness the two most common of these fades matrix supported and either inversely graded facies 1a or nongraded 1b are absent from the closest modem analog mount st helens msh washington as are three others matrix supported normally graded 3 clast supported inversely graded 5 and clast supported nongraded 7a one common mount dutton fades matrix supported inversely to normally graded 4 is the peak flow overbank and transition lahar fades at msh one less common mount dutton lahar fades clast supported inversely to normally graded 6 is the sole facies or channel facies at msh mount dutton facies 2 thin matrix supported nongraded is present at msh as the lahar runout facies plots of maximum particle size mps versus bed thickness bth show significant positive correlations for seven fades as expected of debris flow deposits analogy with msh lahars suggests that mount dutton lahars were cohesionless debris flows grainflows that formed when turbulent floods or pyroclastic surges bulked by eroding material from their bed dispersive pressure produced inverse grading where shearing occurred inverse grading throughout beds is inherited from transient shearing episodes normal grading may be inherited from graded suspensions developed during bulking of turbulent flows or may reflect progressive deposition rather than frictional or cohesive freezing of a laterally graded flow slopes and intercepts of correlation lines for individual facies on an mps bth plot form natural groups three groups comprising facies 5 and 7a 1a and 1b and 3 and 4 differ chiefly in the intercept of the correlation line with the mps axis this intercept indicates thickness independent strength the result of cohesion and possibly other factors all three groups are stronger in this respect by a factor of 2 to 100 than a sampling of ancient nonvolcanic debris flows these three groups correspond to diamictite subfacies of nonvolcanic debris flows from the permian of colorado facies 6 is unlike other fades of the mount dutton formation but similar to ancient nonvolcanic debris flows in its strength properties it resembles whaleback bars that formed during nonuniform flow in the north fork lahars at msh two mount dutton fades 2 and 7b clast supported normally graded do not have a significant positive correlation of mps and bth they are probably deposits of hyper concentrated fluids not lahars in the strict sense

regional stratigraphy sedimentology and tectonic significance of oligocene miocene sedimentary and volcanic rocks northern baja california mexico abstract upper oligocene to middle miocene sedimentary and volcanic rocks in northern baja california were deposited along the western margin of north america during subduction of the guadalupe plate and southward migration of the rivera triple junction regional mapping and compilation of stratigraphic data reveal a sequence of three regionally traceable stratigraphic units 1 oligocene to lower miocene mesa formation basal quartz rich fluvial sandstone grus conglomerate and accessory facies whose detrital compositions reflect the composition of local pre tertiary basement rock 2 lower to middle miocene comondu formation laterally variable sequence of volcaniclastic conglomerate breccia sandstone tuff and minor volcanic flow units 3 widespread mesa capping rhyolite tuff typically welded and crystal rich probably upper miocene in age the mesa formation overlies a highly irregular and deeply dissected erosional surface developed on pre tertiary basement rock the shift from pre mesa erosion to widespread though localized deposition and valley filling records the final phase of late cretaceous to middle tertiary regional subsidence and eastward transgression that resulted from slow cooling and thermal contraction of cretaceous arc crust during a temporal gap in magmatic activity along the western cordilleran margin nonmarine sediments of the mesa formation were deposited in small steep walled paleovalleys and basins that gradually filled and evolved to form through going low energy ephemeral stream systems the gradational upward transition from the mesa to comondu formation records the early to middle miocene onset of subduction related arc magmatism in eastern baja california and related westward progradation of alluvial volcaniclastic aprons shed from high standing eruptive volcanic centers pre existing streams were choked with the new influx of volcanic detritus causing the onset of rapid sediment deposition by stream flows and dilute to viscous sediment gravity flows deposits of the comondu formation thin and fine systematically westward from proximal volcanic conglomerate and breccia with thin basalt and andesite flows in the east to distal volcaniclastic fluvial sandstone in the west these proximal distal relationships help to define the location and paleogeography of active arc flanking volcaniclastic alluvial aprons of the miocene magmatic arc in northern baja california a substantial late miocene drop in regional base level relative sea level is best attributed to regional uplift caused by the renewal of magmatic and thermal activity in northern baja california which has continued to the present day

oblique rifting at oceanic ridges relationship between spreading and stretching directions from earthquake focal mechanisms the relationship between spreading and stretching directions is investigated at oblique spreading oceanic ridges using earthquake focal mechanisms the stretching direction at ridge axes corresponds to the direction of the greatest principal strain 31 taken as the mean trend of the seismic t axes of extensional earthquake focal mechanisms it is compared with the spreading direction provided by global plate motion models we find that the stretching direction trends approximately halfway between the spreading direction and the normal to the ridge trend a result in line with analogue experiments of oblique rifting this result is satisfactorily accounted for with an analytical model of oblique rifting for which the direction of 31 is calculated with respect to rifting obliquity for different amounts of stretching using continuum mechanics for low stretching factors typical of incremental seismic deformations 31 obliquity is two times lower than rifting obliquity for higher stretching factors the stretching and spreading directions become parallel 2006 elsevier ltd all rights reserved

the relationship between late cenozoic tectonic events and stress field and basin development in northeast japan the regional tectonic stress field basin development and crustal deformation of the ne japan arc in the interval between 32 ma to the quaternary can be synthesized based on dike vein and fault orientation data as well as on the compilation of the regional geology an extensional stress field became prevalent from 32 ma and major normal faulting started at 25 20 ma which resulted from back arc rifting normal faulting trending nearly parallel to the arc propagated from the present japan sea coast to the forearc side following the trenchward migration of the main volcanic field from 20 to 15 ma normal faults with an oblique trend to the arc developed due to the counterclockwise rotation of ne japan the rapid clockwise rotation of sw japan since 16 ma produced a nw se directed transtensional stress regime in the ne japan arc due to crustal stretching associated with this pull apart movement the back arc side of the ne japan arc subsided rapidly to middle bathyal environments after the termination of the opening of the japan sea at about 14 ma a neutral stress regime prevailed which included phases of both weak extension and compression lithospheric cooling eventually led to thermal subsidence of the back arc region and igneous underplating caused uplift of the axial zone of the volcanic arc the increase in velocity of the westward motion of the pacific plate at around 4 ma produced strong compression across the arc reactivating most of the miocene normal faults and uplifting the volcanic arc the greatest crustal shortening occurred in areas that were stretched the most in the miocene within the volcanic arc which implies tectonic inversion

the influence of explosive volcanism on fluvial sedimentation the deschutes formation neogene in central oregon abstract the upper miocene to lower pliocene deschutes formation of central oregon is a sequence of interstratified volcanic and sedimentary rocks as much as 700 m thick largely derived from the adjacent cascade range the formation is correlative with the early high cascade eruptive episode that included voluminous pyroclastic eruptions and culminated in the development of an intra arc graben in the central oregon high cascades facies associations and paleocurrent data define three depositional settings 1 the eastward sloping arc adjacent alluvial plain was the depositional site by sheetfloods debris flows and hyperconcentrated flood flows their deposits from 10 to 70 m thick sequences separated by erosion surfaces with up to 60 m of relief widespread superimposed paleosols overlie these facies and constitute the upper 10 to 50 m of the section sedimentation was episodic with pulses of rapid aggradation corresponding to periods of explosive volcanism and abundant sediment supply separated by periods of nondeposition when streams incised to reestablish grade as sediment loads diminished 2 a gravel bedload river flowed northward through the basin at the foot of the alluvial plain and is represented by channel fill conglomerates and overbank sandstones and siltstones 3 a thin sequence derived from older tertiary highlands was deposited on the east side of the basin and was little affected by cascade volcanism extensive development of thick superimposed paleosols reflects the inactive nature of the eastern basin margin as a sediment source deschutes formation deposition records the response of a fluvial system to large volcanism induced sediment loads popular facies models do not adequately allow for the influence of volcanism on fluvial systems because sediment is mobilized episodically and on a scale unparalleled in nonvolcanic settings

structure and development of the changliangshan ductile shear zone north tibet implications for the initial closure of the paleo tethys ocean in the central qiangtang region the changliangshan ductile shear zone extends 80 km in an east west direction in central qiangtang tibetan plateau structural and kinematic analyses show that the shear zone comprises mainly protomylonite mylonite and phyllite with dextral dominated strike slip motion in the northern branch of the shear zone and sinistral strike slip shear in the southern branch concordant u pb ages on the metamorphic rims of zircons from mylonite indicate that deformation along the shear zone started at 249 1 6 7 ma during the initial collision between the north and south qiangtang terranes the 40ar 39ar ages of sericite 221 215 ma combined with available kinematic and structural data indicate that the metamorphic rocks of the shear zone experienced transpressional exhumation and uplift during the late triassic in addition the e w trending changliangshan ductile shear zone marks the contact between yangtze type warm water and gondwana type cold water deposits the initial ne sw directed collision gave rise to shearing along the changliangshan ductile shear zone under lower to middle greenschist facies conditions during the early middle triassic subsequently the yangtze type sediments passively pierced the gondwana type deposits as a wedge shaped unit along the shear zone furthermore the data reveal that the changliangshan shear zone formed during oblique convergence between the north and south qiangtang terranes during initial closure of the paleo tethys

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

analog models of oblique rifting in a cold lithosphere new lithospheric analog models of oblique rifting presented here capture the main characteristics of natural oblique rifts and provide insights into the fault evolution basin segmentation and mantle exhumation occurring during rift localization we present two models one with a preexisting oblique lithospheric weakness model b and another with no weakness zone model a both oblique rifts have an obliquity of about 40 the main results are as follows 1 the fault populations especially during the early stages of deformation are composed of faults that in strike are largely intermediate between rift parallel and perpendicular to displacement this fault population is characteristic of oblique rifts observed in previous studies 2 in later stages faults parallel to the rift become numerous in both models buoyancy forces related to thickness variations in the lithosphere during rift localization play a significant role and control the initiation of rift parallel faults 3 during the final stages of extension in model b the crust is deformed by rift parallel faults while in the basins the small scale deformation pattern is composed of displacement normal faults however in model a displacement normal faults tend to accommodate most of the extension controlling its final stages they probably also control the formation of the ocean continent transition any possible mantle exhumation as well as the geometry of oceanic accretion centers these results provide an insight into the possible evolution of the gulf of aden conjugate margins which developed in an oblique context and most probably without any preexisting rift parallel localizing heterogeneity in the lithosphere

the influence of ephemeral processes on pyroclastic sedimentation in a rift basin volcaniclastic alluvial sequence espanola basin new mexico abstract the plio pleistocene puye formation north central new mexico is a coarse grained conglomeratic wedge of primary and reworked volcanic deposits that prograded eastward into an actively aggrading floodbasin of the central rio grande rift this alluvial system grew in response to the degradation of dacitic and rhyolitic lava domes in the northeastern part of the jemez mountains volcanic field analysis of puye deposits shows that fan sedimentation was governed by the complex interaction of explosive volcanism rift basin tectonics and the highly variable localized ephemeral processes common to arid region alluvial systems preserved in this well exposed sequence are complex associations between primary pyroclastic facies related to explosive eruptions from lava domes and reworked tuffs that record the rapid emplacement of poorly consolidated pumice and ash the sporadic availability of water in this semi arid region and the episodic flood frequency and runoff that accompanied individual eruptions largely controlled puye facies characteristics important among the combined influences of explosive eruptions and ephemeral processes were 1 fluvial transport and reworking of pumice sequences 2 remobilization of loosely consolidated volcanic talus and tuffs by mass flow and 3 marked downslope changes in mass flow deposits and fallout tephra emplaced into lakes

the fluvial and pyroclastic deposits of the cagayan basin northern luzon philippines an example of non marine volcaniclastic sedimentation in an interarc basin the cagayan basin of northern luzon an interarc basin 250 km long and 80 km wide contains a 900 m thick sequence of plio pleistocene fluvial and pyroclastic deposits these deposits are divided into two formations the ilagan and awidon mesa and three lithofacies associations the facies which are interpreted as meandering stream braided stream lahar and pyroclastic flow and fall deposits occur in a coarsening upward sequence meandering stream deposits interbedded with tuffs are overlain by braided stream deposits interbedded with coarser pyroclastic deposits lahars and ignimbrites the coarsening upward volcaniclastic deposits reflect the tectonic and volcanic evolution of the adjacent cordillera central volcanic arc uplift of the arc resulted in the progradation of coarser clastics further into the basin the development of an alluvial fan and migration of the basin depocentre away from the arc the coarsening of the pyroclastic deposits reflects the development of a more proximal calc alkaline volcanic belt in the maturing volcanic arc the cagayan basin sediments serve as an example of the type and sequence of non marine volcaniclastic sediments that may form in other interarc basins this is because the tectonic and volcanic processes which controlled sedimentation in the cagayan basin also affect other arc systems and will therefore control or significantly influence volcaniclastic sedimentation in other interarc basins