paleomagnetic study of some cretaceous and tertiary sedimentary rocks of the klamath mountains province california paleomagnetic investigation of cretaceous outliers and tertiary sedimentary strata of the klamath mountains province and of onlapping cretaceous strata has shown the rocks to be largely remagnetized samples studied are from the upper jurassic to upper cretaceous great valley sequence upper cretaceous hornbrook formation eocene montgomery creek formation and oligocene weaverville formation cretaceous samples that survived the remagnetization have a mean remanence direction that is very close to the expected direction of the cretaceous magnetic field at the locality of the klamath mountains data from both primary and remagnetized samples suggest the possibility of 11 5 15 8 of post cretaceous clockwise rotation of the klamath mountains province none of the data from either the primary or remagnetized samples shows evidence of the large amounts 70 of clockwise rotation that other workers have measured for the lower tertiary of the oregon coast range our data indicate that the oregon coast range and klamath mountains province did not behave as a single rigid block during the early tertiary they also suggest that any post oligocene rotation of the klamath mountains province is less than the approximately 30 post oligocene rotation recently proposed for a combined oregon coast range klamath mountains cascade range block

geochronology of type santacrucian middle tertiary land mammal age patagonia argentina mammal bearing lacustrine and tuffaceous sediments from three localities of the santa cruz formation type fauna of the santacrucian land mammal age in patagonia southern argentina are calibrated by radioisotope dating with the aid of magnetostratigraphy the strata range from about 17 6 ma to perhaps 16 0 ma and are thus of late early miocene age the santacrucian land mammal age ranges from about 18 0 ma to about 15 0 ma

insar observations of 2007 tanzania rifting episode reveal mixed fault and dyke extension in an immature continental rift summary in the early stages of continental rifting extension takes place by normal faulting while in mature continental rifts dyke intrusion dominates little is known about the nature of the transition between fault controlled and dyke controlled extension or about the processes in an intermediate setting here we present observations of the temporal and spatial evolution of surface displacements during the 2007 july 14 august 4 rifting episode in northern tanzania an immature section of the east african rift the ground deformation initiated with subsidence that can be attributed to 40 cm of normal motion on a ne striking fault following july 17 deformation was dominated by the intrusion of 7 km long dyke dyke opening increased gradually to a total of 2 4 m from july 21 the collapse of a shallow graben above the fault dominated the near field displacements comparison to the 2007 dabbahu dyke afar which occurred in a more mature rift shows an order of magnitude scale difference in dyke length using numerical models of dyke propagation we attribute this to the size and depth of the magma chamber in immature rifts the thick crust and slow spreading rate favour small deep magma chambers forming short buried dykes whereas in mature rifts the thinner crust and faster spreading rate favour large shallow magma chambers and long erupting dykes observing the pattern of active processes in the east african rift is key to understanding the development of rift systems and passive margins elsewhere

volcano geodesy and magma dynamics in iceland abstract here we review the achievements of volcano geodesy in iceland during the last 15 years extensive measurements of crustal deformation have been conducted using a variety of geodetic techniques including leveling electronic distance measurements campaign and continuous global positioning system gps geodesy and interferometric analysis of synthetic aperture radar images insar results from these measurements provide a comprehensive view of the behavior of icelandic volcanoes between inflation intrusion and eruption episodes volcanoes are likely to deflate or show no sign of seismic activity subsidence rates are often in the range of a few millimeters to a few centimeters a year reducing progressively with time since the last eruption or intrusion at the volcano subsidence can be caused by cooling and contraction of magma outflow of magma it can be related to plate spreading volcano subsidence or lack of deformation is often interrupted by episodic magma flow towards near surface locations such magma recharge has been observed geodetically at hengill hekla eyjafjallajokull katla grimsvotn and krafla volcanoes with inflow inferred to last from a few months up to two decades in the last 15 years five volcanic eruptions three intrusive events and two m6 earthquakes have occurred in recent years the grimsvotn and katla volcanoes have exhibited continuous inflation of a few centimeters per year which at grimsvotn culminated in an eruption on 1 november 2004 hekla and torfajokull volcanoes have inflated at rates an order of magnitude less subsidence is occurring presently at the askja and krafla volcanoes within the period of geodetic measurement signals consistent with no deformation are typical for most of the 35 active volcanoes in iceland

the pyrope coesite rocks and their country rocks at parigi dora maira massif western alps detailed petrography mineral chemistry and pt path both the coarse and fine grained varieties of the partly coesite bearing pyrope quartzites their interlayered jadeite kyanite rocks and the biotite phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents which were also studied analytically by microprobe prior to pyrope and coesite growth the mg rich metapelites were talc kyanite chlorite rutile ellenbergerite schists which upon continued prograde metamorphism developed first pyrope megacrysts in silica deficient local environments at the expense of chlorite talc kyanite and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc kyanite based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations a prograde pt path simplified for water activity 1 is constructed which passes through the approximate pt conditions 16 kbar and 560 c 29 kbar and 720 c and finally up to 37 kbar at about 800 c where the mg rich metapelite was a pyrope coesite rock with phengite kyanite and talc still present during the retrograde path pyrope was altered metasomatically either into phlogopite kyanite quartz or at a later stage to chlorite muscovite quartz both assemblages yield pt constraints the latter about 7 9 kbar 500 600 c the country rock gneisses have also endured high pressures of at least 15 kbar but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies about 5 kbar 450 c microprobe data are presented for the following minerals pyrope ellenbergerite dumortierite unusually mgti rich jadeite vermiculite formed after na phlogopite paragonite and for several generations of phengite chlorite talc phlogopite dravite and glaucophane in the high pressure rocks as well as for biotite chlorite phengites epidote garnet albite and k feldspar in the country rock gneisses an outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their pt stability limits

mineralogy and phase relations in the blueschist facies of the black butte and ball rock areas northern california coast ranges microprobe study of minerals in rocks of the blueschist facies from the black butte and ball rock areas of the south fork mountain schist northern california coast ranges is the basis in this report for analysis of reactions and phase relations among lawsonite pumpellyite epidote and associated minerals observed assemblages are intermediate in a progression from those observed elsewhere in high p low t schists of the franciscan e 9 panoche pass where lawsonite is present in nearly all lithologies to the relatively low phigh t assemblages of the shuksan blueschist washington where epidote is ubiquitous two successive reactions in this sequence in order of increasing temperature are l lawsonite na amphibole epidote quartz albite chlorite hzo 2 na amphibole pumpellyite quartz epidote ca amphibole albite chlorite h2o the pressure of metamorphism for the black butte and ball rock areas is estimated from the jadeite content of pyroxene and from the stability of aragonite to have been approximately 7 kbar temperature estimates are not tightly constrained by mineral equilibria but are suggested to be in the range of 250 to 300 c

provenance of franciscan graywackes in coastal california a systematic comparison of available detrital modes for graywacke sandstones of the franciscan subduction complex and for coeval sandstones of the great valley sequence in the california coast ranges indicates that both were apparently derived from the same general sources the inferred provenance terrane was the ancestral sierran klamath magmatic arc from which mixed volcanic and plutonic detritus readily entered the adjacent great valley forearc basin at intervals along the trend of the arc trench system arc derived detritus also bypassed the forearc region through submarine canyons that fed the franciscan trench longitudinal flow along both the great valley trough and the franciscan trench achieved wide dispersal of the turbidite sediment suites of both franciscan and great valley samples include an array of subquartzose compositions ranging from feldspatholithic to lithofeldspathic mean framework modes of 17 franciscan suites comprising 203 individual samples and of 23 great valley suites comprising 410 individual samples range from 14 to 44 quartz grains 15 to 54 feldspar grains and 7 to 71 total lithic fragments the ratio of quartz to feldspar remains relatively constant as the proportion of lithic fragments changes the compositional variations reflect differences mainly in the admixture of lithic fragments derived principally from volcanic cover with quartz and feldspar derived principally from erosion of underlying plutons despite major overlap in the compositions of the two sets of samples some franciscan sandstones are somewhat more feldspathic and less lithic than any known great valley counterparts and were probably derived from segments of the arc terrane where exposures of plutons were more extensive than within typical great valley sources higher proportions of non volcanic to volcanic lithic fragments in some franciscan sandstones probably reflect complex recycling processes on the trench slope diagenetic effects in many franciscan suites include apparent wholesale replacement of k feldspar by albite present age control is inadequate to test fully for time dependent trends in the compositions of franciscan sandstones analogous to the known stratigraphic variations in the composition of great valley sandstones this question ought to be investigated in future studies

paleoseismology and seismic hazards evaluations in extensional volcanic terrains extensional structures in volcanic terrains are the surface expression of shallow dike intrusion and can be misinterpreted as structures associated with major tectonic faults dike induced structures can be distinguished from their tectonic counterparts by their association with cogenetic volcanic rocks and by several geometric relationships tensile fissures with little or no vertical displacement fissure swarms flexural monoclines and normal faults are commonly symmetrically distributed about a central eruptive fissure sometimes forming a graben above shallow dikes the structures typically occur within broad zones not narrow belts near a main fault zone reflecting their origin by repeated dike injection colluvial wedges containing records of single large earthquakes generally do not form instead fault scarps with several meter vertical displacements in volcanic bedrock may reflect the cumulative effects of many decimeter scale displacement events from several dike injection episodes the mechanism of dike intrusion and the nature of cointrusive seismicity have important implications for determination of the maximum magnitude and recurrence of earthquakes in extensional volcanic terrains observational seismicity from volcanic rift zones worldwide suggests the maximum magnitudes of dike induced earthquakes are 3 8 0 8 earthquakes are generally small to moderate because downdip extents of faults and fissures are controlled by the depth to the top of the associated dike usually 5 km permitting only small rupture areas also rupture and displacement on faults and fissures migrate incrementally at about the velocity of propagating dikes 0 5 m s as dike dilation stresses the zone above and ahead of the dike earthquake recurrence is tied to recurrence of volcanic cycles based on the geochronology of the associated volcanic materials based on these concepts innovative approaches have led to estimates of the maximum magnitude and recurrence for magma induced seismicity in eastern snake river plain esrp volcanic rift zones the most conservative approach uses surface length fault width downdip extent and rupture areas of normal faults and fissures produced by dike injection to estimate a maximum m5 5 earthquake for an episode of rift zone volcanism chronology of volcanic rocks suggests annual probabilities of 10 4 to 10 5 for volcanic rift zones near idaho national engineering laboratory facilities probabilistic assessments show that ground motion hazard due to volcanic rift zone seismicity is lower than the hazard from other earthquake sources this is because of the relatively low magnitudes and long recurrence intervals of cointrusive earthquakes in volcanic rift zones

accretionary tectonics of the north american cordillera continental geology stands on the threshold of a change that is likely to be as fundamental as plate tectonic theory was for marine geology ongoing seismic reflection investigations into the deep crustal structure of north america are verifying that orogenic zones are underlain by low angle faults of regional extent brown et al 1981 the growing body of regional field relations is likewise delineating numerous orogenic sutures that bound discrete crustal fragments paleomagnetic and paleobiogeographic studies are revealing major latitudinal shifts and rotations within and between suture bounded fragments particularly within the north american cordillera such interdisciplinary studies are leading to a consensus that the cordillera has been built by progressive tectonic addition of crustal fragments along the continent edge in mesozoic and early cenozoic time such crustal growth is referred to as accretionary tectonics in this paper we review some of the important concepts in accretionary tectonics discuss the nature of the materials accreted between central alaska and southern california in jurassic and cretaceous time and consider the general relations between cordilleran accretion and the movement of lithospheric plates the concept of continents growing by peripheral accretion through geologic time has long been a topic of great interest with the advent of plate tectonics a number of different mechanisms for crustal accretion have arisen along with mechanisms for crustal attrition accretion mechanisms include the growth of imbricated sedimentary prisms along inner trench walls slicing off of submarine topographic irregularities within subducting plates and collision of continents and volcanic arcs by ocean basin closure tectonic attrition mechanisms include rifting transform faulting and strike slip or underthrust removal of inner trench wall materials coincident with or in place of accretionary prism growth growth of intraorogenic ocean basins by seafloor spreading is an additional important mechanism for creating accretionary materials as well as displacing crustal fragments an important implication of plate kinematic theory is the likelihood for accretionary and attritionary mechanics to operate in series both in time and space along continental margins since attrition by nature leaves little material evidence of having operated one of the major problems confronting cordilleran geologists lies in the recognition of such attrition within the ancient record particularly when interspersed with accretionary events the spectrum of accretion and attrition mechanisms viewed at cm yr 1 plate transport rates over time scales of 100 m y leads one to suspect a highly mobile history for continental margin orogens the serial arrangement of subducting transform and rifting links along the modern cordillera plate juncture system and both serial and parallel arrangements in the western pacific systems show the complex interplay of such mechanisms through space similar arrangements overprinted through time are suggested by the rock assemblages and structural patterns within the cordillera which presently resemble a collage of crustal fragments davis et al 1978 recognition of the structural state of this collage geologic field mapping and geophysical investigations will bring about a new level of understanding in the growth of continental crust and the reading of stratigraphic records within the fragments and future palinspastic restorations will lead to a new level of understanding in paleogeography and earth history the first problem to be considered is the recognition of native north american crust from exotic fragments that have been accreted to its edge

large pressure gap between the coastal and central franciscan belts northern and central california abstract the nature of the metamorphism of the central belt of the franciscan complex has long been debated did the whole central belt or its melange matrix undergo high p t metamorphism to solve this problem we have investigated the regional distribution of aragonite in these rocks in northern and central california we have discovered more than 300 new localities of aragonite using the staining method by friedman 1959 aragonite occurs as network veins or pods in both metagraywacke and matrix forming metashale and is invariably partially replaced by calcite on its rim thus we conclude that aragonite is stable in all parts of the franciscan central belt from the lower boundary between the coastal and central belts to the topmost part of the central belt and that both the matrix forming melange shale and exotic blocks of the central belt suffered high p t metamorphism we traced the boundary between the central and coastal belts in northern california using the staining method along the gently eastward dipping thrust plane between the underlying coastal and overlying central belts more than 2 5 kbar pressure break is obtained through the lawsonite laumontite aragonite quartz relations such an observed large pressure gap was formed during the exhumation process of the central belt of the franciscan complex

phanerozoic continental accretion and the metamorphic evolution of northern and central california abstract the late paleozoic and younger tectonic fabric of california north of the transverse ranges consists of accreted exotic and parautochthonous lithic belts which extend north south and northwest southeast separated from one another by subvertical strike slip faults and gently to steeply east dipping thrust faults where recognizable folds in general verge to the west and stratigraphic tops face east a rough but systematic westward decrease in geologic age and metamorphic intensity may also be recognized exotic terranes of uncertain provenance include the central metamorphic belt of the klamath mountains and the salina sur obispo composite block of the central coast ranges pre permian sections of the eastern klamath terrane and portions of the sierran calaveras melones complex may also represent far traveled crustal entities allochthonous oceanic basement seamounts and pelagic sediments are additional transported materials widespread in most of the parautochthonous lithotectonic belts that make up much of the rest of northern and central california ophiolitic assemblages are especially abundant along the fault zones which mark boundaries between these units the parautochthonous lithotectonic belts appear to have been constructed in the vicinity of the continental margin and or an offshore arc judging from the nature and provenance of their constituent igneous and sedimentary rocks intimately associated tholeiitic calc alkaline complexes indicate that at least locally ophiolite generation occurred nearby the salinia sur obispo amalgamated terrane exhibits the effects of fragmentation and removal of crust now represented by the suture juxtaposing the two blocks other areas may have been subjected to similar truncations but relationships are not as clear far traveled terranes are characterized by ages of formation and recrystallization foreign to surrounding units the nature of the protoliths and grades of metamorphism developed within them contrast with those of juxtaposed lithotectonic belts on the other hand essentially in situ growth of many californian lithotectonic belts is suggested by a gradually decreasing age of formation and both monotonically decreasing times and grades of metamorphism proceeding towards the pacific basin some parautochthonous units as well as exotic terranes are locally bounded by discontinuous blueschist belts the latter attesting to at least episodic preservation of the effects of subduction metamorphism and offloading into the accreting continental margin phanerozoic evolution of northern and central california thus apparently consisted of a more or less in situ seaward encroachment of the continental margin relative to the craton involving occasional truncation and loss of section with a concomitant semi continuous or episodic incorporation of allochthonous seamounts oceanic crust and deep sea sediments and the less frequent arrival and juxtaposition of exotic microcontinental fragments of problematic but distant source

characteristics and genesis mechanism of ground fissures in taiyuan basin northern china abstract since the 1980s ground fissures in the taiyuan basin have experienced blowouts resulting in a significant but hidden danger to cities in the basin this paper describes the plane and damage characteristics of the ground fissures based on the findings of field investigations and mapping most ground fissures in the basin were determined as linear with pinch out reappearances on the surface they were characterized by both horizontal tension with maximum widths and vertical offsets of up to 40 and 15 cm respectively the ground fissure strikes in the basin were observed to be consistent with those of nearby active faults and the long axis direction of the groundwater depression funnel the roads and houses that lie across the fissures were determined to be the most affected by ground fissure disasters in the basin the primary damage modes for the roads were vertical offset and surface collapse whereas buildings experienced foundation failure and tensile damage profiles obtained through geological drilling and trench exploration further revealed obvious tension and synsedimentary characteristics of the ground fissures correlated with existing faults correlation analysis shows that the formation of ground fissures in taiyuan basin is mainly related to faults and excessive groundwater extraction theoretical analysis revealed that the size and activity of ground fissures in the basin were positively correlated with four factors the dip of faults in an area regional tensile stress difference between the thicknesses of aquifers on either side of a fault and groundwater exploitation the significant increase in ground fissure activity since the 1980s mainly reflects uncontrolled groundwater discharge hence prevention of excessive groundwater exploitation in the basin would mitigate the destructive effects of the ground fissures

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

metamorphic evolution of the central metamorphic belt klamath province california an inverted metamorphic gradient beneath the trinity peridotite metabasalts and metasedimentary rocks of the devonian central metamorphic belt comprise the lower plate of the east dipping trinity thrust system in the klamath province an inverted metamorphic gradient is preserved in the central metamorphic belt metamorphic conditions decrease from amphibolite facies adjacent to the trinity thrust through albite epidote amphibolite facies to upper greenschist facies at the base of the central metamorphic belt mineral chemistry mineral assemblages and limited geothermometry suggest that peak metamorphic conditions decrease structurally downward from 650 50 c at the trinity thrust to 500 50 c at the base of the central metamorphic belt under pressures of 5 3 kbar synmetamorphic ab qtz veins up to 1 m thick increase in abundance towards the trinity thrust infiltration of h2o co2 fluids derived from prograde devolatilization reactions in the central metamorphic belt caused extensive hydration and metasomatism of the trinity peridotite the hanging wall block of the trinity thrust zone geological relationships and the preserved inverted metamorphic gradient suggest that the central metamorphic belt formed in an east dipping devonian subduction zone in an oceanic environment the central metamorphic belt appears to represent a discrete slice of accreted oceanic crust several km thick rather than progressively accreted material metamorphic pressures recorded by the central metamorphic belt are intermediate between the 2 kbar pressures recorded in dynamothermal aureoles beneath obducted ophiolites and the 7 10 kbar preserved in subduction related inverted metamorphic gradients the lack of blueschist facies mineral assemblages in the central metamorphic belt may possibly be explained by an anomalously warm geotherm prior to subduction or early shear heating prior to the arrival of wet rocks at depth

post middle miocene accretion of franciscan rocks northwestern california deformed sedimentary rocks assigned to the franciscan assemblage in the king range south of cape mendocino northern california are dominantly deep water argillite and sandstone occurring as thick to thin bedded locally channelized marine turbidites of arkosic to andesitic volcani clastic composition these rocks structurally overlie late cretaceous coniacian to campanian basaltic pillow flows flow breccias diabase sills and arkosic sandstone turbidites that also are assignable to the franciscan assemblage and are well exposed along the coast at point delgada radiolarians and planktic and benthic foraminifers indicate that the oldest rocks in the king range may be early tertiary and that the youngest rocks are middle miocene or younger the youngest rocks yet reported from the franciscan assemblage partly coeval miocene relizian and luisian to quaternary rocks representing shallower sedimentary facies of the inner shelf to outer continental slope occupy the eel river basin and isolated fault bounded slivers adjacent to the king range on the north and east wide zones of penetrative shearing that may delineate a middle miocene or younger suture separate the deep water franciscan rocks of the king range from less severely deformed neogene rocks of the eel river basin the king range appears to be a displaced terrane of oceanic basement overlain by paleogene and neogene sedimentary and igneous rocks of continental and oceanic derivation this terrane may have been displaced north or eastward and obductively accreted to california 15 m y ago before or perhaps during northward passage of the pacific farallon north american plate junction

mesozoic evolution of exotic terranes and marginal seas western north america abstract regional terrane analysis has been combined with a global evaluation of plate kinematics to produce a new tectonic model for the mesozoic evolution of western north america and its associated marginal seas the model employs a two tiered data reliability ranking system to resolve conflicts within data sets the lower tier of the ranking system involves the assignment of a numerical rank to paleomagnetic and or paleobiogeographic data based on each data set s reliability the upper tier of the ranking system places the paleomagnetic and paleobiogeographic data in perspective by assigning a relative order of importance to different types of data the most reliable data are considered to be departure rift and arrival collision times that are tightly constrained by independent data sets evidence of subduction and or strike slip motion also ranks high in the master ranking system paleomagnetic and vertebrate paleobiogeographic data come next in the hierarchy followed by invertebrate and floral paleobiogeographic data application of this approach to a case study the baja british columbia controversy has resulted in a coherent model for the entire region results derived from the application of this modelling approach to the problem of the mesozoic assembly of western north america suggest that its evolution may be modeled using a minimum of 57 large terranes oblique convergence may have been the dominant tectonic process along the western north american margin during the mesozoic causing large scale latitudinal transport of many exotic terranes the motions of the terranes produced a number of marginal seas yukonia etc that were short lived but important components of western margin tectonics a combination of collision events side swiping episodes and changes in plate kinematics led to the initiation of several regional tectonic upheavals including the nevadan and sevier orogenies

controls on the geometry of a holocene crater row a field study from southwest iceland the reykjanes peninsula rift zone in southwest iceland is a highly oblique segment of the mid atlantic ridge system which accommodates nw se extension during rifting episodes that consist of eruptions and normal faulting and e w left lateral shear strain along strike slip faults during longer amagmatic periods dominant tectonic features on the peninsula are a series of generally ne striking sub parallel eruptive fissures and normal faults and a cross cutting zone of n s striking right lateral strike slip faults the last series of rifting episodes ended in 1227 and a proposed 1 000 year cyclicity predicts the start of a new series of eruptions within the next 200 years in order to more accurately characterize the nature of eruptions on the reykjanes peninsula we present a new spatially accurate map of the 2 350 year old sundhnukur crater row in the western part of the peninsula which was examined in detail in order to determine the structural controls on crater row geometry and to understand the interactions that take place between eruptive fissures and pre existing geological structures volcanism is sometimes influenced by small perturbations in the surroundings such as gravitational loading topography changes in crustal properties or the presence of fault zones but there are few field examples showing how fissures are influenced by these pre existing structures we identify 27 fissure segments ranging in strike from 006 to 053 with varying spacing and overlap between them significant local variability in strike and stepping sense of segments occurs in proximity to topographic highs as well as within zones of faulting that pre date the crater row strike also varies at the northern end of the crater row as it approaches a region of older crust at the rift margin our data support numerical and laboratory modeling results which show that local topography pre existing fractures and crustal properties influence the path taken by magma on its way through the shallow crust

the ophiolitic north fork terrane in the salmon river region central klamath mountains california the north fork terrane is an assemblage of ophiolitic and other oceanic volcanic and sedimentary rocks that has been internally imbricated and folded the ophiolitic rocks form a north trending belt through the central part of the region and consist of a disrupted sequence of homogeneous gabbro diabase massive to pillowed basalt and interleaved tectonitic harzburgite u pb zircon age data on a plagiogranite pod from the gabbroic unit indicate that at least this part of the igneous sequence is late paleozoic in age the ophiolitic belt is flanked on either side by mafic volcanic and volcaniclastic rocks limestone bedded chert and argillite most of the chert is triassic including much of late triassic age but chert with uncertain stratigraphic relations at one locality is permian the strata flanking the east side of the ophiolitic belt face eastward and depositional contacts between units are for the most part preserved the strata on the west side of the ophiolitic belt are more highly disrupted than those on the east side contain chert argillite melange and have unproven stratigraphic relation to either the ophiolitic rocks or the eastern strata rocks of the north fork terrane do not show widespread evidence of penetrative deformation at elevated temperatures except an early tectonitic fabric in the harzburgite slip fiber foliation in serpentinite phacoidal foliation in chert and mafic rocks scaly foliation in argillite and mesoscopic folds in bedded chert are consistent with an interpretation of large scale anti formal folding of the terrane about a north south hinge found along the ophiolitic belt but other structural interpretations are tenable the age of folding of north fork rocks is constrained by the involvement of triassic and younger cherts and crosscutting late jurassic plutons deformation in the north fork terrane must have spanned a short period of time because the terrane is bounded structurally above and below by middle or late jurassic thrust faults the north fork terrane appears to contain no arc volcanic rocks or arc derived detritus suggesting that it neither constituted the base for an arc nor was in a basinal setting adjacent to an arc sediment source details of the progressive accretion and evolutionary relationship of the north fork to other terranes of the klamath mountains are not yet clear

accretion of the franciscan complex attending jurassic cretaceous geotectonic development of northern and central california scattered igneous activity marks the triassic margin of california but an andean volcanic plutonic arc began to form in the sierra nevada white inyo range and klamath mountains at ca 170 ma during underflow of the oceanic lithosphere this calc alkaline belt supplied volcanogenic erosional debris to the mariposa galice proximal overlap sequence by ca 165 160 ma transpression also generated high pressure low temperature basaltic eclogites garnet blueschists and amphibolites along the middle and late jurassic convergent plate junction at ca 170 155 ma most of these returned surfaceward as tectonic blocks only during the mid cretaceous the high grade metamafic metamorphosed mafic blocks that formed during construction of the emergent arc and derived mariposa galice strata predate the onset of franciscan trench deposition near the end of jurassic time the klamath salient evidently migrated 200 km westward an earliest cretaceous westward stepout of the convergent junction apparently formed directly offshore from the klamath imbricate orogen but to the south preexisting oceanic crust capped lithosphere was trapped landward as the coast range ophiolite andean arc detritus began to accumulate on this ophiolitic basement within the great valley forearc and outboard franciscan oceanic trench at ca 145 140 ma because earliest cretaceous relatively continuous great valley strata were deposited on the stable north american plate protected from both surface and subcrustal erosion forearc terrigenous sedimentation also signaled coeval deposition in the outboard trench 10 25 m y after middle and late jurassic initiation of the continental margin calc alkaline arc voluminous sedimentation and the accretion of franciscan and san joaquin sections of great valley rocks took place during mid and late cretaceous time after a flare up in igneous arc activity at 125 120 ma the youngest sierran granites have ca 85 ma ages reflecting extinction of the magmagenic zone beneath northern and central california and presumably reflecting subhorizontal oceanic plate underflow attending the laramide orogeny feebly zeolitized upper cretaceous and lower tertiary franciscan coastal belt rocks derived from the now inactive arc underwent only low pressure burial metamorphism reflecting shallow offloading and or large scale transpression they were not appreciably subducted although coastal belt units are members of the franciscan lithotectonic assemblage their origin contrasts with the rest of the subduction complex

surface deformation and strike slip faulting controlled by dyking and host rock lithology a compendium from the krafla rift iceland abstract in the present work we have obtained a detailed complete view of surface deformation at a volcano tectonic structure belonging to the krafla fissure swarm ne iceland through the integration of field mapping and the aerial structure from motion technique asfm as well as through orthomosaics and digital surface models dsms the documented 2 5 km long deformation zone is marked by a range of features such as topographic bulging parallel extension fractures and narrow grabens affected by local floor uplift which we have explained as the effect of the northward shallow propagation of a dyke from the krafla magma chamber our lithostratigraphic field survey of the area at a 1 5000 scale indicates that major changes in the pattern of surface deformation take place across the contacts between deposits with different rheological properties the transition from very stiff lavas to softer hyaloclastites produces changes from extensional fracturing to normal faulting though the width of the deformation zone does not change we have also documented a number of ne sw left lateral and nnw sse right lateral strike slip fault zones that are rotated in clockwise and anticlockwise sense to the main nne ssw graben trend and extend outwards as much as about 17 m the graben zone terminates to the north by splitting into two strike slip fault zones these were likely generated at the front of the dyke tip during its propagation and were later on bypassed by the advancement of the dyke as the dyke entered the soft hyaloclastites propagation was slowed down and the dyke became thicker

structural evidence for northeastward movement on the chocolate mountains thrust southeasternmost california the late cretaceous chocolate mountains thrust of southeastern california and southwestern arizona places a block of proterozoic and mesozoic continental crust over the late mesozoic continental margin oceanic sedimentary and volcanic rocks of the regionally distinctive orocopia schist the chocolate mountains thrust is interpreted as a thrust burial subduction fault rather than a low angle normal exhumation unroofing uplift fault an important parameter required to understand the tectonic significance of the chocolate mountains and related thrusts is their sense of movement the chocolate mountains thrust zone contains sparse to locally abundant mesoscopic asymmetric folds fabric relations supported by regional geologic evidence indicate that these folds are an integral part of and coeval with the thrust zone on a lower hemisphere equal area plot representing the orientation and sense of asymmetry of 80 thrust zone folds from 36 localities spread over an area 60 by 10 km z folds plot northwest of and s folds plot southeast of a northeast southwest striking vertical plane of overall monoclinic symmetry the only sense of movement consistent with the collective asymmetry of the thrust zone folds is top to the northeast asymmetric microstructures studied at several localities also indicate top to the northeast movement paleomagnetic data suggest that the original sense of thrusting prior to neogene vertical axis tectonic rotation related to the san andreas fault system was northward the essential point is that movement of the upper plate of the chocolate mountains thrust evidently was continentward continentward thrusting suggests a tectonic scenario in which an insular or peninsular microcontinental fragment collided with mainland southern california the suture predicted by this model is elusive but the chocolate mountains thrust and underlying orocopia schist themselves may represent the suture at the present level of exposure alternative tectonic models involving subduction of the orocopia schist eastward beneath continental southern california circumvent the suture problem but are presently not supported by any direct structural evidence

overview of ground fissure research in china ground fissures in china have garnered increasing international attention in recent years due to their areal extensiveness large scale and high hazard potential these fissures especially those in the fen wei basin and north china plain severely restrict planning and construction in big cities threaten the operation of underground and above ground railways and cause serious economic loss recent research shows that ground fissures in china occur mainly in sedimentary basins and develop along active faults and at the edges of groundwater cones of depression most of these fissures are caused by the combined action of tectonic stress and hydrodynamic force effective ground fissure disaster reduction technologies are developed and they include pumping control reasonable avoidance adaption to deformation and local strengthening this paper provides an overview on ground fissure studies in china and demonstrates how research results can serve economic development and geohazard mitigation

regional detrital zircon provenance of exotic metasandstone blocks eastern hayfork terrane western paleozoic and triassic belt klamath mountains california abstract the addition of oceanic material through accretion underplating was an important process in construction of the convergent transpressive cordilleran margin of north america according to radiolarian ages the late triassic or younger eastern hayfork terrane represents trench deposits that accreted outboard of the coeval north fork immature island arc it consists chiefly of chert and argillite but contains tectonic and olistostromal blocks of preexisting rock units derived from exotic sources that include inboard klamath terranes fine grained terrigenous debris was shed from these eastern terranes but the eastern hayfork melange also contains blocks of quartzofeldspathic metasandstone of uncertain provenance new detrital zircon u pb ages from five blocks of immature variably metamorphosed arkosic to subarkosic sandstone range from late proterozoic to late archean with modes of 1804 1843 1970 2300 and 2550 2585 ma the adjacent inboard north fork terrane contains zircons with phanero

jurassic to miocene k ar dates from eastern central patagonian cordillera plutons chile 45 48 s thirty nine k ar and one ar ar radiometric dates from the eastern central meso cenozoic patagonian batholith and eastern satellite plutons of the aysen region of southern chile between latitudes 45 and 48 s combined with previous dating of seven plutons have yielded six age groups 1 middle to late jurassic 2 early cretaceous 3 mid cretaceous 4 late cretaceous 5 oligocene and 6 miocene in general the cretaceous and younger ages correspond to previous reported ages for other parts of the main batholith but for the satellite plutons to the east show a wider age spectrum than the previously accepted late miocene dates these results indicate a relatively continuous late jurassic to mid cretaceous plutonism known to have been coeval with volcanic activity followed by intermittent magmatism biotite k ar dates of c 143 151 and 106 109 ma from cataclastic granitoids may be marking the time of deformation a review of all radiometric data on magmatic rocks from the region between 45 and 48 s in chile shows a gap in palaeocene ages that may correlate with a period of low angle flat slab subduction between 65 50 ma

metamorphism of a late jurassic volcano plutonic arc northern california usa the metamorphic history of the middle to upper jurassic volcanic and hypabyssal rocks exposed in the klamath mountains and sierra nevada of california is related in part to the rifting of a volcano plutonic arc the callovian to kimmeridgian rocks exposed in the region consist of from north west to south east a back arc ophiolite a rifted volcanic arc and a volcanic arc complex all of these units have been metamorphosed and contain various combinations of the phases chlorite amphibole epidote prehnite and pumpellyite projection of coexisting phases onto the composition plane mgo mgo feo and al2o3 fe2o3 0 75 cao na2o through quartz water albite and epidote results in consistent mineralogical compatibilities within each region but crossing tie lines between regions this suggests that the volcanic and hypabyssal rocks from each region have equilibrated under different intensive conditions the back arc ophiolite in the north has suffered subseafloor high t p hydrothermal metamorphism with geothermal gradients on the order of 100 c km 1 the rifted volcanic arc has undergone synchronous burial hydrothermal and contact metamorphism metamorphic field gradients in the region pass through the prehnite pumpellyite and greenschist facies suggesting geothermal gradients on the order of 30 c km 1 the southernmost volcanic arc complexes contain metavolcanics of the pumpellyiteactinolite and greenschist facies suggesting moderate to high p t metamorphism and geothermal gradients on the order of 20 c km 1 the apparent increase in rifting and calculated geothermal gradients from south east to north west suggest that the observed very low and low grade metamorphism may be a response to enhanced thermal gradients during extension of the volcanic arc this correlation between the extent of rifting and metamorphism is consistent with a model of diastathermal metamorphism of a propagating rift along the western margin of north america during the late jurassic the plate tectonic setting may be analogous to the present day andaman sea region

surface deformation at the krafla volcano north iceland 1982 1992 extensive measurements of ground deformation at the krafla volcano iceland have been made since the beginning in 1975 of a series of eruptions and intrusions into the fissure system that extends north and south of the volcano i concentrate on measurements before and after the eruption of september 1984 the last event of this series when the largest volume of lava was erupted the patterns of ground deformation associated with the 1984 eruption determined by precision levelling electronic distance measurements and lake level observations were similar to earlier intrusions and eruptions in that the surface of the volcano subsided and the fissure system widened as magma moved laterally from a shallow central reservoir into the fissure system the shallow magma reservoir of krafla continued to expand for about five years after the eruption but a slow subsidence of the central area began in 1989 besides the presence of an inflating and deflating shallow magma reservoir at a depth of 2 5 km beneath the krafla caldera another inflating magma reservoir may exist at much greater depth below krafla the accumulation of compressive strain by numerous rift intrusions and eruptions since 1975 along the flanks of the north south krafla fissure swarm is being released slowly and will probably be reflected in the results of deformation measurements near krafla for the next several decades the total horizontal extension of the krafla rift system in 1975 1984 was about 9 m equal to about 500 years of constant plate divergence the extension is twice the accumulated divergence since previous rifting events and eruptions in 1724 1729

a middle jurassic to early tertiary andean sierran arc model for the coast belt of british columbia regional tectonic features of the western canadian cordillera can be interpreted in terms of middle jurassic accretion of a single composite superterrane stikinia wrangellia alexander to ancestral north america closure of the intervening cache creek bridge river ocean moved the continental edge to a new position west of the accreted superterrane the coast belt is primarily a succession of superimposed middle jurassic to early tertiary magmatic arcs related to prolonged subduction of pacific ocean lithosphere beneath the new north american margin discrete magmatic pulses separated by lulls or periods of relative quiescence successively overprinted the new margin the insular and intermontane superterranes previously viewed as widely separate entities prior to mid cretaceous time were already amalgamated before an initial middle jurassic overprint produced an ancestral coast belt late jurassic to early cretaceous rift related marine basins mid to late cretaceous compressional structures and early tertiary extensional features coincident with the coast belt are subsidiary intraplate attributes that reflect external adjustments in plate motions within a primary subduction related andean magmatic arc

metamorphic evolution of two different eclogites in the franciscan complex california u s a the eclogites in california u s a have been regarded as tectonic blocks which formed 150 160 ma ago and have been tectonically incorporated into younger rocks of the franciscan complex of 90 130 ma however in ward creek northern california two different types of eclogites were identified type i is a typical tectonic block and type ii is a possible in situ eclogite formed during the franciscan metamorphism most eclogites in the franciscan complex are type i eclogites type i eclogites in ward creek consist of garnet omphacite epidote glaucophane phengite rutile barroisite quartz and have coarse grained gneissic texture they have precursors of the epidote amphibolite facies and are overprinted in various degrees by blueschist facies retrograde metamorphism remnants of metamorphic rinds which consist of actinolite talc chlorite and formed during metasomatic interaction with serpentinizing peridotite occur at the margins of type i eclogite blocks type ii eclogites are medium grained and consist of garnet chloromelaniticclinopyroxene epidote glaucophane paragonite phengite rutile quartz petrochemical characters of type ii eclogites are similar to those of in situ eclogitic schists in the ward creek metabasite sequence epidote amphibolite f facies precursors metamorphic rinds and overprint of retrogressive blueschist facies metamorphism are not found in type ii eclogites garnets in type ii eclogites are medium grained unaltered and idioblastic and have higher spessartine contents sp14 25 whereas garnets in type i eclogites are coarse grained and chloritized and have lower spessartine contents sp2 7 the pt conditions of peak metamorphism for type ii eclogites are 380 10 11 5 kb type i eclogites follow a counter clockwise pt path from the epidote amphibolite through eclogite to blueschist facies whereas the presumable in situ type ii eclogites follow a clockwise pt path from the blueschist to eclogite facies

hydrofracturing stress measurements in the iceland research drilling project drill hole at reydarfjordur iceland two independent suites of hydrofracturing stress measurements were conducted in the top 600 m of the iceland research drilling project deep hole at reydarfjordur east iceland as indicated by the continuously extracted drill core the tested section consists of tertiary subaerial tholeiitic lava flows cut by many basaltic dikes the density of the basalt was used to estimate the vertical stress v in megapascals at depth d in meters v 0 027d in the 33 to 270 m depth range the horizontal principal stresses hmax and hmin as calculated from hydrofracturing results increase gradually with depth but at a lower rate than the vertical stress hmin 2 1 0 016d hmax 3 3 0 016d in the 300 to 550 m depth range a major perturbation occurs with both horizontal stress magnitudes rapidly rising between 300 and 400 m and then uniformly decreasing between 400 and 550 m to a level apparently predictable by the relationships obtained in the top 270 m conditions favoring normal faulting v hmax hmin were established in the 540 to 580 m range and are conjectured to persist at greater depths since the vertical stress gradient is higher than that of hmax and hmin this would be in accord with the predominant fault type in east iceland the relative low horizontal stress values and the small difference between them are supportive of ocean floor spreading theories and contradict near surface high compressive stresses previously determined by hast 1973 in east iceland the large increase in horizontal stress in the range of 300 to 500 m depth is not understood a correlation appears to exist between the zone of almost unintruded thick lava flows 200 500 m and that of high stresses an intuitive model suggesting locked in stresses in this zone is proposed the hydrofracturing determined overall mean direction of hmax is n50 e while a more selective sampling of results yields n40 e the measured stress directions appear to be reasonably compatible with the local structure such as dike direction nne and lava dip wsw significantly hmax direction is subparallel to the n20 e trend of the nearest fissure swarms in the axial rift zone this almost coincidence of directions like the measured stress magnitudes is also in agreement with the model of spreading lithospheric plates being pulled apart

age origin and significance of an unconformity that predates island arc volcanism in the northern sierra nevada the paleogeographic position of rocks of the northern sierra nevada relative to north america has long been the topic of debate a detailed study of the lakes basin region of the northern sierra nevada substantiates that the shoo fly complex of early paleozoic age was folded and unconformably overlain by an island arc sequence in late paleozoic time microfossils recovered during the study represent the first in situ fauna ever collected from the shoo fly complex and provide an ordovician silurian lower limit on the early deformation late jurassic nevadan deformation strongly affected all rocks in the region regional considerations suggest that mid paleozoic deformation may have been widespread in the sierra nevada and could indicate an early phase of an eastward prograding suture that ultimately resulted in emplacement of the roberts mountain allochthon of the great basin

permo triassic accretionary subduction complex southwestern klamath mountains northern california the western paleozoic and triassic subprovince of the southwestern klamath mountains has previously been subdivided into three relatively steeply dipping fault bounded litho tectonic terranes in descending structural order from east to west these terranes are 1 the north fork terrane 2 the hayfork terrane and 3 the rattlesnake creek terrane in a general fashion each of these terranes exhibits its own particular set of lithologic structural and temporal relationships that sets it apart from adjacent terranes this threefold subdivision of the southwestern klamath mountains needs modification only as far as the hayfork terrane is concerned the hayfork terrane is actually a composite terrane consisting of a structurally lower assemblage of predominantly basaltic metavolcaniclastic rocks and a structurally higher assemblage of chaotically disrupted metasedimentary rocks juxtaposed along a previously unrecognized major regional thrust these two lithologically and structurally distinct assemblages have been elevated to the status of a terrane in this report and are herein referred to as the western hayfork terrane structurally lower metavolcaniclastic assemblage and the eastern hayfork terrane structurally overlying metasedimentary assemblage and the fault which juxtaposes these two terranes is herein referred to as the wilson point thrust this paper will focus on the eastern hayfork and north fork terranes and their relationship to other roughly coeval tectonic elements of the klamath mountain province the eastern hayfork terrane is best described as a tectono stratigraphic unit consisting of a complex chert argillite and quartzose sandstone melange and broken formation that contains radiolarian cherts as young as late triassic in age scattered throughout this terrane are various types of blocks some of which are clearly exotic metachert is by far the most common type of metamorphic block and these blocks appear to have been derived via sedimentary processes from the paleozoic central metamorphic belt and or the mesozoic stuart fork formation shallow water permian limestone blocks are also plentiful and are interpreted as reefal accumulations which flanked seamounts that became incorporated into the melange and broken formation as proto pacific ocean floor was subducted beneath the western margin of north america during at least the permo triassic to early jurassic the north fork terrane has at its base basaltic rocks tentatively interpreted as an oceanic crustal remnant generated within an open ocean ridge setting that was removed from any appreciable source of continental or volcanic arc derived detritus overlying the basaltic rocks is a sequence of pelagic hemipelagic terrigenous and tuffaceous metasedimentary rocks that have yielded a late triassic and early or middle jurassic radiolarian fauna the metasedimentary rocks of the north fork terrane are lithologically very similar to those of the eastern hayfork terrane both terranes consist predominantly of chert and argillite contain blocks of metamorphic rock and permian limestone and are at least in part of overlapping age however the metasedimentary rocks of the north fork terrane are structurally coherent unlike the chaotic melange and broken formation of the eastern hayfork terrane taken together the north fork and eastern hayfork terranes bear remarkable similarities to both modern and ancient rock assemblages that occupy sites within zones of lithospheric plate convergence and are interpreted as tectonic elements of an underthrust plate margin developed oceanward of a volcanic arc represented by the permo triassic to middle jurassic volcanic strata of the eastern klamath belt

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

submarine and superimposed contact metamorphic oxygen isotopic exchange in an oceanic arc sawyers bar area central klamath mountains california usa new bulk rock oxygen isotope data indicate a complicated history of fluid rock interactions in the upper few kilometers of a basaltic arc attending its mesozoic accretion to the western margin of north america folded multiply recrystallized weakly metasomatized mafic volcanics and interstratified sediments of the sawyers bar terrane an eastern segment of the western triassic and paleozoic belt were investigated the following scenario can now be reconstructed 1 island arc tholeiites iats ocean island basalts oibs and distal turbidites were deposited in a subsea environment during permian and early mesozoic time 170 245 ma basaltic rocks underwent low temperature alteration by seawater recrystallization occurred at 100 200 c and 1 kbar alkali exchange and variable mg enrichment were accompanied by increases in bulk rock 18o values of the greenstones from 6 to approximately 10 o preceeding initial stages of island arc formation 2 middle jurassic 165 170 ma suturing of the seaward oceanic arc structurally beneath a landward 227 ma blueschist terrane resulted in regional deformation and greenschist facies metamorphism pervasive overprinting took place without important chemical or isotopic exchange under conditions of 300 425 c 3 1 kbar 3 granitoid plutons emplaced during late middle jurassic time 160 165 ma heated adjacent wallrocks to 500 600 c at pressures of approximately 2 3 kbar thermal upgrading resulted in devolatilization of isotopically heavy metasediments and in the exchange of high 18o fluids with intercalated greenstones 18o values in iat and oib metavolcanics increased from 9 to 10 along axial portions of ns trending folds to more than 15 o where metabasalts are intimately interlayered with the metasediments construction of this sector of the continental margin through the assembly of a weakly deformed and polymetamorphosed oceanic arc was attended by a sequence of aqueous fluid rock isotopic exchanges reflecting the p t time evolution of the complex

volcanism and the dynamics of open magma chambers the repeated eruption of many volcanoes is now accounted for by events taking place in a subterranean magma chamber which is open in the sense that the inflow of material to the chamber from below leads to a magma pressure p which exceeds the lithostatic pressure pl by an amount greater than the tensile strength of the chamber walls before the contents of the magma chamber have reached thermodynamic and geochemical equilibrium this is the condition which must be satisfied when an eruption occurs we have now derived a relationship giving the critical volume of added material v required to cause an eruption as a function of magma compressibility and the volume by which the chamber expands

oceanic ridge crest processes oceanic ridge crest processes constitute a vigorous frontier of multidisciplinary research with global ramifications in the earth atmospheric and biologic sciences this deep ocean frontier is gradually opening in response to investigations which are elucidating the role of these processes in the lithosphere hydrosphere atmosphere and biosphere and the application of advanced instrumental and interpretive techniques which are increasing the quality and quantity of data these techniques include crustal drilling with borehole logging deep sea drilling project reports high resolution bathymetry sea beam etc side scan sonar seamarc i and ii gloria etc imagery with video and photography multichannel seismics determination of water column properties ctd with rosette sampler etc and submersible applications

contemporary holocene and quaternary deformation of the asal rift djibouti implications for the mechanics of slow spreading ridges because the frequency and character of rifting events along mid ocean ridges are largely unknown how the repetition of such events gives rise to rift structures is unexplored the asal rift in the afar depression of djibouti africa provides the world s best subaerial analogue for young slow spreading mid ocean ridges seismic geodetic and field observations of a seismovolcanic event in 1978 at asal yield estimates of the fault and dike locations geometry displacement and volume of basalt extruded in a rifting event a 6 9 kyr old lake shore highstand at asal has been warped downward by 70 m providing a holocene measure of the vertical deformation across the rift the rift topography furnishes an older datum which we infer to be 34 6 kyr old using the holocene deformation rate we find that faults throughout the rift valley are active holocene slip rates diminish beyond 4 km from the rift axis late quaternary rates decrease beyond 6 7 km the holocene slip rates are used to estimate repeat times by taking the displacement on the faults which slipped in 1978 as characteristic we find tectonic events on individual faults recur every 200 300 years half the rift faults slipped together in the 1978 event if this is typical then groups of faults are activated every 100 150 years we suggest that half the events take place in the rift axis accompanied by volcanic extrusion the remainder occur peripheral to the neovolcanic zone and involve fault slip only both events having a repeat time of 200 300 years given the 10 km width of the rift and its 16 mm yr 1 spreading rate the mean age of the material in the rift should be 350 kyr an order of magnitude older than the inferred age of the formation of the rift topography the subsidence rate of the rift axis during the past 35 kyr is 8 9 mm yr 1 with the rate of infilling by volcanic extrusion 1 mm yr 1 the resulting net subsidence rate about equal to the half spreading rate of the rift could not be sustained for 300 kyr without significant infilling by lavas thus both observations suggest that the long term vertical deformation in the rift has not been steady state instead we suggest that there is a rifting filling cycle at asal with the most recent filling episode ending 35 kyr

significance of mid mesozoic peridotitic to dioritic intrusive complexes klamath mountains western sierra nevada california a mid mesozoic plutonic suite ranging from ultramafic gabbroic rocks to dioritic rocks and commonly intruded by younger granitoids is widespread in the klamath mountains western sierra nevada california the ultramafic gabbroic rocks are clinopyroxene rich and commonly vary from wehrlite to olivine hornblende clinopyroxenite and melagabbro associated dioritic rocks include biotite two pyroxene diorite monzodiorite and the granitic rocks typically range from tonalite to granodiorite these plutonic complexes are coextensive and broadly coeval with a suite of weakly metamorphosed volcanic rocks ranging in composition from basalt to basaltic andesite to andesite the volcanic rocks are chiefly volcaniclastic and are characterized by several distinct phenocryst assemblages clinopyroxene plagioclase olivine or hornblende the plutonic complexes and the associated volcanic rocks are restricted to terranes that lack continental sialic crust and that at least locally contain juvenile ophiolitic crust generated adjacent to the locus of magmatism thus their petrogenesis may be related to recurrent fracturing and magmatism within a rifted ensimatic arc

time relations and structural stratigraphic patterns in ophiolite accretion west central klamath mountains california new geochronological data and published structural and stratigraphic data show that two distinctly different ophiolitic assemblages formed in general proximity to one another at nearly the same time and were subsequently imbricated along a regional thrust zone the josephine ophiolite constitutes a complete oceanic crust and upper mantle sequence which lies within the western jurassic belt of the klamath province within the study area the josephine ophiolite was formed by seafloor spreading at about 157 m y before present it was immediately covered by a thin pelagic and hemipelagic sequence which grades into a thick flysch sequence both of which comprise the galice formation the galice flysch was derived from volcanic arc and uplifted continental margin orogenic assemblages a major nonvolcanic source for the galice flysch appears to have been the western paleozoic and triassic belt of the klamath province exposed to the east proximal volcanic arc activity migrated to the site of the josephine galice section by 151 m y and is represented by numerous dikes and sills which intrude the ophiolite and galice formation the preston peak ophiolite is a polygenetic assemblage consisting of 1 a pre mid jurassic tectonitic peridotite amphibolite substrate which represents disrupted and unroofed basement of the western paleozoic and triassic belt and 2 an upper mafic complex which was intruded through and constructed above the tectonite substrate at about 160 m y the mafic complex consists primarily of diabase hypabyssal rocks that are overlain by diabase clast breccia and hemipelagic deposits a major arc plutonic complex was emplaced into the preston peak ophiolite in at least two pulses at 153 and 149 m y major phases of this complex consist of wehrlite gabbro pyroxene diorite and hornblende diorite the josephine ophiolite is interpreted as the remnants of interarc basin crust the preston peak ophiolite is interpreted as either a primitive remnant arc complex or a rift edge facies for the josephine interarc basin the galice formation represents a submarine fan complex that was built on juvenile crust of the josephine basin floor during the time interval of 153 to 149 m y the locus or arc magmatism migrated to an area which included the interarc basin floor and the remnant arc or basin edge the basin shortly thereafter closed by convergent tectonics during the nevadan orogeny resulting in the imbrication of the josephine and preston peak ophiolites and their superimposed arc assemblages the transition from seafloor spreading generation of josephine ophiolite to its tectonic accretion by convergence and basin closure occurred within 5 to 10 m y the process of rifting and ophiolite formation in series with convergence and ophiolite accretion is considered an important mechanism for generating and displacing allocthonous terranes in the klamath mountains sierra nevada region and perhaps throughout the western cordillera

imbricate low angle faulting in uppermost franciscan rocks south yolla bolly area northern california the south yolla bolly area of northern california contains the intersection of the franciscan complex the klamath mountains and the great valley sequence four distinct lithic units among them the south fork mountain schist as well as several subunits have been discerned in the franciscan rocks of this area these units are composed mostly of coherent or semi coherent layered sedimentary and mafic igneous rocks and their metamorphic equivalents rather than melange and are arranged in largely upright parallel imbricate fault slices four major low angle faults have been clearly recognized paleontologic data indicate that at least one fault places older material over younger lithic units exhibit various degrees of differences in metamorphic mineral assemblages textural grade and amount of internal deformation across the faults with more intensely deformed and metamorphosed material occupying successively higher structural positions these contrasts indicate that some of these faults served as surfaces by which deformed rocks were emplaced differentially upward during and or after metamorphism original bedding and compositional layering in all of the coherent units generally parallel the thrust faults two fault bounded units are each composed of a coherent sequence of basalt at the base overlain by bedded chert and further overlain by mudstone and graywacke or their metamorphic equivalents the faults which underlie these two sequences seem to have originated as decollements localized at or near a sedimentary rock mafic crust interface perhaps as discrete underthrust slices the distinct contrasts in original rock composition among these units plus the aforementioned characteristics seem to indicate that these faults had an early history in the accretionary prism apart from their later use as convenient surfaces for differential upward emplacement of metamorphosed material

oceanic plateaus the fragmentation of continents and mountain building many anomalous rises in today s oceans may be submerged continental fragments detached from previous continents ancient island arcs or basaltic piles formed by hot spots and spreading centers these rises are embedded in their respective moving oceanic plates and are fated to be consumed at active margins where such rises are being consumed at present e g the nazca ridge they cause cessation of volcanism disruption of the downgoing slab and possible shifts in plate boundary configuration many past rises including numerous continental fragments have been recognized within mountain belts as allochthonous terranes they constitute a large portion of the orogenic belts in the north pacific from mexico through western north america alaska east siberia japan and in new zealand the orogenic deformation in these belts is possibly the result of the accretion of the allochthonous terranes many terranes have been accreted with substantial deformation also in the alpine chain well before major continent continent collisions it is suggested therefore that the accretion of fragments may be the common process of the deformation phase of mountain building subduction of normal oceanic crust may be insufficient for deformation whereas full continent continent collision may not be necessary the general validity of this conclusion depends critically on whether allochthonous terranes caused orogenic deformation in the andes or not most of the accreted fragments with continental affinities in the mesozoic cenozoic orogenic belts of the world can be traced back to the breakup of gondwana beginning with a pacifica domain in the permian through a larger india domain in the early mesozoic and continuing through the separation of the somalia plate in the near future the reasons for this 250 million year breakup process are not known but some kind of thermal process possible of mantle wide scale is implied

analysis and comparison of two types of ground fissures in dali county in the weihe basin china dali county is located in the eastern weihe basin where frequent ground fissure disasters occur dali ground fissures are macroscopically part of the ground fissure group in the weihe basin and have caused serious damage to infrastructure and agricultural production in this area in this paper 25 ground fissures in dali county are identified through investigation and two typical ground fissures in fengcun and weizhuang are selected for research through field investigation trench excavation geophysical exploration soil water content tests and collapsibility coefficient tests the basic features shallow structural characteristics deep structural characteristics and genetic mechanisms of the two ground fissures are revealed the strike of the fengcun ground fissure is 45 almost parallel to the shuangquan linyi fault the length is 2 km and the profile is ladder like and extends deep with synsedimentary characteristics the weizhuang ground fissures are circular in plan view with a length of 315 m and trumpet shaped in profile they disappear in the paleosol layer the mechanism of the fengcun ground fissure is related to basin extension and fault activity the ground fissures are the surface reflections of deep faults the mechanism of the weizhuang ground fissures is related to loess collapsibility and seepage the malan loess has differential settlement in collapsible and noncollapsible areas and ground fissures are the boundary line for differential settlement of loess finally based on the genetic mechanisms of ground fissures dali ground fissures can be divided into two types tectonic and nontectonic

polygenetic ophiolite belt of the california sierra nevada geochronological and tectonostratigraphic development the assumption that ophiolite sequences are generated at essentially one point in geologic time by the process of sea floor spreading is critical for modern concepts in the tectonics of ophiolites and for topics dealing with their structure and petrology however this assumption has only been verified in a few locations by an integrated geochronological and structural stratigraphic approach many ophiolite sections are reconstructed from structurally disrupted sequences with the idealized ocean floor model in mind such reconstructions are prone to error without adequate age control on each of the reconstructed fragments this is a significant problem in structurally complex regions where more than one generation of ophiolite may be present in this paper new pb u zircon ages are presented for key locations along a 375 km segment of the western sierra nevada ophiolite belt these age data are combined with structural stratigraphic observations and published ages and significant tectonic implications for the ophiolite belt emerge three different ophiolitic assemblages are recognized with igneous ages of about 300 200 and 160 m y b p rocks of the 300 m y assemblage are in a completely disrupted array of metamorphic tectonite slabs and serpentinite matrix melange fragments of upper paleozoic seamounts occur in association with the ophiolitic melange and together these assemblages constitute the basement framework for the western sierra pb u and k ar isotopic systematics are complex within this framework and indicate a polymetamorphic history systematics in the 200 and 160 m y assemblages are less complex and give tighter igneous age constraints rocks of the 200 m y assemblage are in a semi intact state with only local tectonite and melange zones rocks of the 160 m y assemblage are intact but nevertheless deformed both the 200 and 160 m y assemblages have equivalent age basinal volcanic sedimentary sequences that lie unconformably above the ophiolitic melange basement in each case the basinal sequences locally extend conformably into the upper stratigraphic levels of the age equivalent ophiolite sections these relations along with vestiges of intrusive contacts between the edges of both younger ophiolites and the melange basement indicate that the younger ophiolites underwent igneous formation in proximity to the melange basement the sierran ophiolite belt is considered to have formed by a multistage process initiated by the early mesozoic tectonic accretion of upper paleozoic sea floor in general proximity to the ancient continental margin regional metamorphism and ophiolitic melange resulted this accretionary nucleus became the basement of jurassic age primitive volcanic arc terranes which underwent rifting episodes during the production of the 200 and 160 m y ophiolites the rifting episodes resulted in the formation of sedimentary basins which were the depositional sites of volcanic sedimentary sequences non volcanic sources for the basinal sedimentary rocks include the melange basement and continental margin terranes contact zones between pre existing basement and the juvenile ophiolitic sequences created during the rifting episodes consist of dynamothermal metamorphic aureoles protoclastic deformation zones and cross cutting dikes such edge zone assemblages are in most localities obscurred or destroyed by superimposed deformations resulting from convergent and perhaps transform motions along basin edges both the 200 and 160 m y basins were destroyed by compressional orogenic episodes shortly after their formational episodes destruction of young ophiolite floored basins may be a common course of events when small oceanic type plates are generated along continental margin environments such tectonic settings are ideal for the emplacement of young ophiolite sheets

characteristics and mechanism of the longyao ground fissure on north china plain china abstract among more than 1000 ground fissures developed over the past 30 years having induced severe geohazards in the north china plain the longyao ground fissure in hebei province is famous for its longest length and causing the worst damage and widest influence in this region in order to reveal the origin of its formation the geological background developmental characteristics and genetic mode of the longyao ground fissure were investigated in detail by geological surveys measurements trenching drilling and seismic exploration this fissure has a total length of 39 km and is characterized by normal fault extension left lateral slip and dominant along polyline distribution these characteristics are well consistent with its underlying fault the nei jv fault the fissure causes the damage to buildings structures along a polyline and in the form of a horizontal tensile crack subsidence and shear failure according to the trenching drilling and shallow seismic profiling this fissure possesses the characteristics of synsedimentary faults reflecting the historical stages of fissure activities regarding its genesis this fissure was generated under the coupling action of multiple factors which took shape after the earthquake and was enlarged under the action of groundwater pumping the regional extension first caused a fracture system in the surface strata and then the action of seismic activities started to control the fracture and finally the pumping action of excessive groundwater and other influential factors promoted the formation of the current ground fissure

episodic rifting and volcanism at krafla in north iceland radon 222 emission from fumaroles near leirhnjukur from june 1978 until late 1980 radon emission from the leirhnjukur fumaroles was monitored within the krafla caldera of north iceland where episodic volcanism is occurring frequent sampling of the fumaroles shows that no easily identifiable short term radon precursors occur in the days prior to subsidence of the caldera despite an observed increase in microseismicity preceding deflation following the onset of subsidence however the radon emission of the fumaroles gradually increases and reaches a maximum 3 6 days later the radon in the fumaroles is assumed to be transported from depth by steam and noncondensing gases that slowly escape from the geothermal water table the cause of the co episodic increase in radon emission appears to be a temporal rise of the water table driven by fissure closure resulting in an abbreviated transport time for radon to the surface fumaroles furthermore the closing of the fissures appears to cause a transient increase in the velocity of transport making the shape of the anomalies broader and higher than is predicted from a change in the water level alone changes in radon emission also coincide with fluctuations in fumarolic activity and permanent changes in the level of geothermal water that occur during periods of uplift

magmatic fissures and their systems in iceland a tectonomagmatic model 1 volcanic systems in iceland comprise a central volcano linked to magmatic fissures whose subsurface architecture remains enigmatic the surface dimensions and trends of active fissures show their spatial relation with fault systems structure of the fremri namur and hengill volcanic systems was analyzed the linear vent arrays marking recent fissures are tightly bound by vertical segmented faults while normal faults accommodating extension are common on the flanks of the fissure swarm evidence of normal frictional slip on the fissure bounded faults suggests that failure occurred in response to subsidence with subsequent dilation this is supported by inward tilting of strata adjacent to faults compatible with a downsag phase we propose that magmatic fissures have vertical feeders with lateral offshoots extending along the rift zone their inflation deflation during an eruptive cycle causes subsidence such magmatically generated faults can be subsequently modified by tectonic extension

character distribution and tectonic significance of accretionary terranes in the central alaska range the central part of the alaska range near mount mckinley is composed of nine separate tectonostratigraphic terranes that were accreted in southern alaska during late mesozoic time these terranes now form long linear fault bounded belts that are subparallel to the denali fault on the north but oblique to the fault on the south the postaccretion right lateral offset along the denali fault system is about 200 km from north to south the major terranes are 1 yukon tanana terrane metasedimentary and metavolcanic rocks mostly undated but including rocks of known late paleozoic age polymetamorphosed with terminal events in late mesozoic 2 pingston terrane isoclinally folded upper triassic deep water silty limestone quartzite and carbonaceous slate folded with upper paleozoic phyllite chert tuff and minor limestone 3 mckinley terrane upper paleozoic flysch chert and minor limestone intruded by large gabbro sills and dikes and overlain by thick piles of triassic porphyritic pillow lava the top of the section is thick sequence of upper mesozoic conglomerate flysch chert and phyllite 4 dillinger terrane very thick sequence of strongly folded lower paleozoic micaceous sandstone turbidites graptolitic shale and deep water limestone locally overlain unconformably by jurassic fossiliferous sandstone or triassic pillow basalt 5 windy terrane heterogeneous assemblage of serpentinite basalt tuff and chert ophiolite with paleozoic and mesozoic flysch and blocks of mid paleozoic fossiliferous limestone 6 mystic terrane predominantly upper paleozoic flysch and conglomerate but also includes lower paleozoic graptolitic shale pillow basalt and shallow water limestone and upper paleozoic fossiliferous limestone sandstone chert and undated pillow basalt 7 chulitna terrane upper devonian ophiolite overlain by upper paleozoic chert volcanic conglomerate limestone and flysch capped by lower triassic limestone and upper triassic red beds basalt and interbedded basalt and limestone later mesozoic rocks are sandstone chert and argillite 8 westfork terrane jurassic chert sandstone conglomerate and triassic and jurassic crystal tuff 9 broad pass terrane upper paleozoic chert tuff and argillite with blocks of devonian and older limestone locally associated with serpentinite these diverse terranes of mixed oceanic and continental affinities are now juxtaposed to form a complex sequence of folded and faulted rootless nappes major suture zones between them are occupied by intensely deformed upper mesozoic flysch all of the terranes differ markedly from wrangellia the southermost coherent accreted terrane in alaska paleomagnetic measurements from triassic rocks prove that wrangellia originated near the equator probably in the southern heimsphere paleolatitudes for terranes in the central alaska range are not yet determined but other lines of evidence biogeographic and lithologic suggest large scale northern transport for some terranes

mechanism of fuping ground fissure in the weihe basin of northwest china fault and rainfall the special geological conditions of the weihe basin located in the southern fenwei graben system are the cause of many of the ground fissures developing in this area the tingzi ground fissure in fuping county is one of the typical and particular ground fissures found in the weihe basin the ground fissure is famous for its sudden appearance after heavy rainfall to explore the characteristics and mechanism of the fissure some geological methods were carried out in the study area for this study such as surveying mapping trenching drilling and seismic exploration the main conclusions are as follows 1 the fissure is located on the hanging side of the secondary fault of the liquan pucheng heyang fault with a ne40 ne75 strike and a length of 2 7 km 2 the fissure is steeply inclined and presents a funnel shape in the shallow strata there are many secondary cracks near the main fissure 3 the fissure is connected with underlying fault and possesses the characteristics of synsedimentary faults reflecting the historical stages of fissure activities 4 the fissure destroys farmlands and houses along the fissure and mainly causes tensile damage 5 the fissure was controlled by regional extension and fault and it was promoted and enlarged by rainfall and other influential factors

weak long lived ground deformation related to iwate volcanism revealed by bayesian decomposition of strain tilt and positioning data abstract weak long lived ground deformation associated with the volcanic unrest of mt iwate northeastern japan in 1998 2002 is detected from borehole strainmeter tiltmeter and global positioning system gps data this deformation is relatively weak 0 01 day strain 0 01 rad day tilt and 0 01 cm day displacement but persists for a period of 5 years to detect such weak long term deformation a new decomposition method based on bayesian theory is developed and applied to the recorded data this method is effective for extracting secular drift from the strain and tilt data and seasonal changes from the gps data from a selection of possible models a composite source model consisting of a dike series and a mogi source is identified as the best fit to the observed data the deformation is divided into 8 phases based on the characteristic variations in tilt and strain based on this model the deformation sequence is considered to have initiated with the intrusion of a dike at intermediated depth below the summit region between february and april 1998 with short periods of shallow dikes intrusion toward the west in march and april shallow dike intrusion resumed in september reaching a final volume of 5 9 10 6 m 3 the mogi source is inferred to have been located 3 km beneath the western flank of the volcano slowly inflating to a maximum volume 1 2 10 7 m 3 by the end of 2000 then deflating by 49 to 2002 this ground deformation was accompanied by seismic migration from the summit region in the vicinity of the intruding dike to the west following the dike extension this dike correlating seismic activity ceased simultaneously with deflation of the mogi source suggesting that the mogi source was a hydrothermal reservoir that was breached by the intruding dike the bayesian decomposition method developed in this study makes it possible to clarify the detailed characteristics of dike intrusions and the long lived activity of the mogi source utilizing the high sensitivity of strain and tiltmeters coupled with the stability of gps

the bending mechanism of anping ground fissure in the hebei plain north china the anping fissure is one typical and particular ground fissure in the north china plain this study was carried out in anping area with geological means such as surveying mapping trenching and drilling the geological structure and disasters were analyzed by both qualitative and quantitative methods the main conclusions are as follows 1 anping ground fissure is the local manifestation of regional tectonic stress field and is basically folding directional hidden and intermittent 2 under the control of shen rao fault anping ground fissure was exposed and expanded by water related human activities and then was cracked by small and medium sized earthquakes 3 the forming process followed the bending cracking rules the uplifting force from deep in the earth bent the entire overlying strata the tangential stress below the ground surface counterbalanced the horizontal stress of soils even breaking through their tensile strength and inducing cracks the fissure was periodic and wavery because the cracking force was proportional to the uplifting force but inversely proportional to the stratigraphic depth

contemporary plate motion and crustal deformation the measurement of relative plate velocities during the past few years is a signal accomplishment in earth science leading to refinement of the precepts of steady motion of plate interiors and cyclic deformation along plate margins regrettably deformation premonitory to an earthquake has yet to be detected with confidence delineation of the spatial and temporal buildup of strain between earthquakes however has put limits on models of the earthquake cycle a diverse set of fault structures and crustal rheologic conditions can explain the pattern of surface strain accumulation and release along strike slip faults in contrast the geometry of thrusts and normal faults revealed by earthquake deformation has been found to differ markedly from expectations geodetic observations have proved vital to monitor the ascent of magma through the earth s crust and to predict volcanic eruptions at the earth s surface episodic vertical and horizontal deformation in southern california remains a subject of dispute if anything it is less certain than it once seemed

episodic rifting and volcanism at krafla in north iceland growth of large ground fissures along the plate boundary the rifting of the plate boundary in north iceland forms new ground fissures and reactivates old ones continuous growth of fissures is observed in the crust above a magma chamber beneath the krafla caldera as it responds to changes in subterranean magma pressure rapid episodes of fissure formation and reactivation are observed in conjunction with magma intrusions at depth along the rift zone that constitutes the plate boundary between the north american and eurasian plates the krafla caldera contains several fissures that open during periods of uplift lasting 100 to 300 days and that close incompletely in subsidence episodes lasting several hours to 3 weeks the average rate of opening differs from one period to another ranging from 0 1 to 0 5 mm d fissure closing and average closing rates also vary from one subsidence episode to another ranging from minimal values of 2 1 mm and 2 3 mm d to maximal values of 49 mm and 18 mm d respectively during the two intrusion episodes of september 1977 and march 1980 the magma migrated toward the south with an average velocity 0 6 and 0 5 m s respectively in the september 1977 episode some fissures in the rift zone widened with a rate greater than 190 mm d whereas during the march episode that probably resulted in a thinner intrusion the maximum widening rate was only 50 mm d the data suggest that the magnitude of a subsidence episode is determined at least partly by the available excess magma pressure in the chamber to explain the extensive growth of open fissures and normal faulting in the rift zone it is necessary to assume that in addition to the intrusion of the main central dike at depth many small dikes are intruded at shallow depths less than 1 km