the role of a keystone fault in triggering the complex el mayor cucapah earthquake rupture large earthquakes can rupture several faults analysis of seismic data from the 2010 el mayor cucapah earthquake in california suggests that multiple faults were pinned to a keystone fault whose rupture triggered cascading slip the 2010 mw 7 2 el mayor cucapah earthquake in baja california mexico activated slip on multiple faults of diverse orientations1 2 which is commonly the case for large earthquakes3 4 5 6 the critical stress level for fault failure depends on fault orientation and is lowest for optimally oriented faults positioned approximately 30 to the greatest principal compressive stress7 yet misoriented faults whose positioning is not conducive to rupture are also common8 9 here we use stress inversions of surface displacement and seismic data to show that the el mayor cucapah earthquake initiated on a fault that owing to its orientation was among those that required the greatest stress for failure although other optimally oriented faults must have reached critical stress earlier in the interseismic period coulomb stress modelling shows that slip on these faults was initially muted because they were pinned held in place by misoriented faults that helped regulate their slip in this way faults of diverse orientations could be maintained at critical stress without destabilizing the network we propose that regional stress build up continues until a misoriented keystone fault reaches its threshold and its failure then spreads spontaneously across the network in a large earthquake our keystone fault hypothesis explains seismogenic failure of severely misoriented faults such as the san andreas fault and the entire class of low angle normal faults

submergence and uplift associated with the giant 1833 sumatran subduction earthquake evidence from coral microatolls the giant sumatran subduction earthquake of 1833 appears as a large emergence event in fossil coral microatolls on the reefs of sumatra s outer arc ridge stratigraphic analysis of these and living microatolls nearby allow us to estimate that 1833 emergence increased trenchward from about 1 to 2 m this pattern and magnitude of uplift are consistent with about 13 m of slip on the subduction interface and suggest a magnitude mw of 8 8 9 2 for the earthquake the fossil microatolls also record rapid submergence in the decades prior to the earthquake with rates increasing trenchward from 5 to 11 mm yr living microatolls show similar rates and a similar pattern the fossil microatolls also record at least two less extensive emergence events in the decades prior to 1833 these observations show that coral microatolls can be useful paleoseismic and paleogeodetic instruments in convergent tectonic environments

holocene paleoseismicity temporal clustering and probabilities of future large m 7 earthquakes on the wasatch fault zone utah the chronology of m 7 paleoearthquakes on the central five segments of the wasatch fault zone wfz is one of the best dated in the world and contains 16 earthquakes in the past 5600 years with an average repeat time of 350 years repeat times for individual segments vary by a factor of 2 and range from about 1200 to 2600 years four of the central five segments ruptured between 620 30 and 1230 60 calendar years b p the remaining segment brigham city segment has not ruptured in the past 2120 100 years comparison of the wfz space time diagram of paleoearthquakes with synthetic paleoseismic histories indicates that the observed temporal clusters and gaps have about an equal probability depending on model assumptions of reflecting random coincidence as opposed to intersegment contagion regional seismicity suggests that for exposure times of 50 and 100 years the probability for an earthquake of m 7 anywhere within the wasatch front region based on a poisson model is 0 16 and 0 30 respectively a fault specific wfz model predicts 50 and 100 year probabilities for an m 7 earthquake on the wfz itself based on a poisson model as 0 13 and 0 25 respectively in contrast segment specific earthquake probabilities that assume quasi periodic recurrence behavior on the weber provo and nephi segments are less 0 01 0 07 in 100 years than the regional or fault specific estimates 0 25 0 30 in 100 years due to the short elapsed times compared to average recurrence intervals on those segments the brigham city and salt lake city segments however have time dependent probabilities that approach or exceed the regional and fault specific probabilities for the salt lake city segment these elevated probabilities are due to the elapsed time being approximately equal to the average late holocene recurrence time for the brigham city segment the elapsed time is significantly longer than the segment specific late holocene recurrence time

subtropical sea level history from coral microatolls in the southern cook islands since 300 ad abstract relative sea level rsl history is reconstructed for the southern cook islands since 300 ad using porites sp coral microatolls the upper surface or height of living coral of the modern microatolls on rarotonga was rigorously determined by 400 laser survey measurements to be constrained by the sea level 0 36 m below mean sea level with a standard deviation of 0 008 m that is equivalent to midway between mean low water neaps and mean low water springs tidal datums fossil microatoll history on the fringing reefs is fragmentary due to multi decadal to centennial windows of microatoll growth cyclone destruction and preservation through coastal progradation the mid late holocene highstand ceased prior to 500 bc and rsl fell from 1 3 0 1 m to 0 45 0 15 m by 1000 ad at a rate of 0 5 mm yr the rsl minimum occurred during the late 1700s to 1800s ad at 0 2 m below present before rising during the 1900s during the past 500 yr rsl fluctuated including an abrupt fall at 1750 ad following a sustained multi decadal high anomaly similarly low rsl anomalies and slow coral growth rates during the 950 to 1000 ad period are indicative of pacific basin wide climate variability and are consistent with extreme la nina like climate anomalies previously reported around the pacific basin the fall in rsl during the first millennium ad crossed over tidal thresholds for rapid coastal progradation when ambient mean high water neaps and mean high water springs fell below the low tide level of the mid late holocene high stand this coastal response is inversely analogous to the projected response to sea level rise during the latter part of the modern century

consistency of teleseismic reporting since 1963 abstract changes in the rate of occurrence of smaller events have been recognized in the rupture zones of upcoming large earthquakes in several postearthquake and one preearthquake study a data set in which a constant portion of the events in any magnitude band are consistently reported through time is crucial for the recognition of seismicity rate changes which are real related to some process change in the earth such a data set is termed a homogeneous data set the consistency of reporting of earthquakes in the noaa hypocenter data file hdf since 1963 is evaluated by examining the cumulative number of events reported as a function of time for the entire world in eight magnitude bands it is assumed that the rate of occurrence of events in the entire world is roughly constant on the time scale examined here because of the great size of the worldwide earthquake production system the rate of reporting of events with magnitudes above m b 4 5 has been constant or increasing since 1963 significant decreases in the number of events reported per month in the magnitude bands below m b 4 4 occurred during 1968 and 1976 these decreases are interpreted as indications of decreases in detection of events for two reasons first they occur at times of constant rates of occurrence and reporting of larger events second the decrease during the late 19609s has also been recognized in the teleseismic data reported by the international seismological centre isc this suggests that the decrease in the number of small events reported was related to facets of the earthquake reporting system which the isc and noaa share the most obvious candidate is the detection system during 1968 detection decreased in the united states central and south america and portions of the south pacific this decrease is probably due to the closure of the vela arrays bmo tfo cpo ubo and wmo during 1976 detection decreased in most of the seismically active regions of the western hemisphere as well as in the region between kamchatka and guam the cause of this detection decrease is unclear these detection decreases seriously affect the amount of homogeneous background period available for the study of teleseismic seismicity rate changes if events below the minimum magnitude of homogeneity are eliminated from the teleseismic data sets the resulting small numbers of events render many regions unsuitable for study many authors have reported seismicity rate decreases as possible precursors to great earthquakes few of these authors have considered detection decreases as possible explanations for their results this analysis indicates that such considerations cannot be avoided in studies of teleseismic data

evidence for seismogenic normal faults at shallow dips in continental rifts several recent observations indicate that normal faulting earthquakes occasionally occur on faults dipping 35 dips often considered shallow most of these occur in the woodlark and aegean rifts the woodlark and aegean rifts are found to generate significantly more earthquakes than others and are the most rapidly extending so display the widest variety of fault behavior even within the woodlark rift system extension rates vary along strike with the shallowest dipping faults confined to the most rapidly rifting segment here several events mw 6 0 6 8 feature nodal planes dipping 23 35 these planes are subparallel to shear zones bounding nearby metamorphic core complexes including one imaged to 8 9 km depth by seismic reflection profiling in the western gulf of corinth at least one large event mw 6 4 occurred on a fault dipping 33 as the woodlark example this rift segment exhibits a high opening rate 10 20 mm yr several other cases elsewhere based on older historic data microseismicity or geologic inference suggest seismic slip at similar or shallower dips still no documented large earthquake exhibits seismic slip on sub horizontal surfaces dip 10 15 stress rotation may explain the 23 35 dips but thus far no realistic mechanism has been found more likely these faults represent surfaces somewhat weaker than surrounding rock through some combination of modest cohesion of the surrounding rock and slightly lower frictional coefficients on the fault such weakening may be a consequence of high slip rates which rapidly generate large offsets and mature fault systems

evidence for paleoseismic slip on a continental low angle normal fault tectonic pseudotachylyte from the west salton detachment fault ca usa abstract the potential of continental low angle normal faults lanf to nucleate large 6 0 m w earthquakes at low angles remains unclear despite much focused research we document evidence for ancient seismicity along a continental lanf detachment fault that formed and slipped at low angles and produced tectonic pseudotachylyte these thick and laterally persistent pseudotachylyte accumulations along the west salton detachment fault wsdf salton trough usa preserve convincing evidence for a frictional melt origin including spherulitic microlites ductile flow structures preservation of high temperature phases as clasts and injection veins cumulative thickness of pseudotachylyte along the fault ranges from 0 1 to 1 5 m and pseudotachylyte cataclasite in the fault core and damage zone are exposed along 2 6 km length of the fault reworked fragments of pseudotachylyte in cataclasites and multiple generations of cataclasites provide evidence for the preservation of multiple earthquake cycles the limited exposure

frictional properties of natural fault gouge from a low angle normal fault panamint valley california 1 we investigate the relationship between frictional strength and clay mineralogy of natural fault gouge from a low angle normal fault in panamint valley california gouge samples were collected from the fault zone at five locations along a north south transect of the range bounding fault system spanning a variety of bedrock lithologies samples were powdered and sheared in the double direct shear configuration at room temperature and humidity the coefficient of friction was measured at a range of normal stresses from 5 to 150 mpa for all samples our results reinforce the intuitive understanding that natural fault gouge zones are inherently heterogeneous samples from a single location exhibit dramatic differences in behavior yet all three were collected within a meter of the fault core for most of the samples friction varies from 0 6 to 0 7 consistent with byerlee s law however samples with greater than 50 wt total clay content were much weaker 0 2 0 4 expandable clay content of the samples ranged from 10 to 40 wt frictional weakness did not correlate with expandable clays our results indicate that friction decreases with increasing total clay content rather than with the abundance of expandable clays the combination of field relations analytical results and friction measurements indicates a positive correlation between clay content fabric intensity and localization of strain in the fault core a mechanism based upon foliations enveloping angular elements to reduce friction is suggested for weakening of fault gouge composed of mixed clay and granular material we provide broad constraints of 1 5 km on the depth of gouge generation and the depth at which fault weakness initiates we show that slip on the panamint valley fault and similar low angle normal faults is mechanically feasible in the mid upper crust if the strength of the fault is limited by weak clay rich fault gouge

earthquake supercycles inferred from sea level changes recorded in the corals of west sumatra records of relative sea level change extracted from corals of the mentawai islands sumatra imply that this 700 kilometer long section of the sunda megathrust has generated broadly similar sequences of great earthquakes about every two centuries for at least the past 700 years the moment magnitude 8 4 earthquake of september 2007 represents the first in a series of large partial failures of the mentawai section that will probably be completed within the next several decades

seismic recurrence intervals and timing of aseismic subduction inferred from emerged corals and reefs of the central vanuatu new hebrides frontal arc the recognition and dating of corals that have been killed by tectonic uplift allow us to date paleoseismic uplifts in the vanuatu island arc we recognize corals that record paleouplifts by their similarity to those known to have died during contemporary sudden uplifts and date them 1 by counting annual coral growth bands only if part of the coral is alive at the time of collection or 2 by newly developed techniques for obtaining 230 th ages by mass spectrometry the mass spectrometric method produces isotopic ages with precisions of 3 to 9 years 2 in the 0 1000 years b p time range the 230 th ages in this time range appear to be accurate samples whose ages are known by counting coral growth bands give 230 th ages that are indistinguishable from their growth band ages by dividing the average increment of uplift for the latest holocene uplifts by the mean holocene uplift rate we can estimate average seismic uplift recurrence intervals for the past 6000 years the results for each of four central vanuatu arc segments are 1 north santo emerged 1 2 m in 1866 a d and 0 6 m 107 years later in 1973 a d the average coseismic uplift of 0 9 m and mean holocene uplift rate of 4 3 mm yr 1 suggest a longer recurrence interval of 212 years 2 south santo emerged 0 29 m in 1946 and 0 26 m 19 years later in 1965 including the related 1971 event here the mean holocene uplift rate is 5 5 mm yr 1 the uplift data suggest a longer average recurrence interval of about 51 years 3 north malekula emerged 1 23 m near 1729 a d and 1 05 m 236 years later in 1965 the mean holocene uplift rate of 2 7 mm yr 1 and mean coseismic uplift of 1 14 m for dated events suggest a longer recurrence interval of 422 years 4 part of southernmost malekula has uplifted continuously or episodically by about 0 35 m from about 1957 until at least mid 1983 a d the maximum uplift of 2 7 mm yr 1 occurs near a nest of small earthquakes both the earthquakes and rapid uplift suggest that interplate slip beneath south malekula may be continuous rather than episodic episodes of 0 35 m uplift would have to recur every 130 years to maintain the 2 7 mm yr 1 uplift rate in contrast we find no evidence of interseismic vertical movements for the other three blocks the most reasonable interpretation of these results is that the seismic recurrence intervals and processes for accommodation of slip are quite different on adjacent arc segments we have used the most widely accepted moment magnitude relationship to evaluate the accumulated seismic slip caused by large earthquakes occurring since 1920 in all four arc segments this analysis suggests that the seismically radiated moments account for less than one third to one tenth of the slip associated with plate convergence the similarity between the paleoseismic record of uplifts and the contemporary record of coseismic uplifts suggests that this analysis can be generalized to times before 1920 for the northern three segments of central vanuatu aseismic slip probably occurs in the same years as large earthquakes because the contemporary coral record records uplifts only in years having large historic earthquakes this suggests that aseismic slip is not continuous and does not occur at rates which vary slowly over the course of the earthquake cycle the south santo segment may have the highest proportion of seismic slip because the mean recurrence interval of 51 years is shortest and the mean holocene uplift rate of at least 5 5 mm yr 1 is the fastest

tectonic uplift mechanism of the goodenough and fergusson island gneiss domes eastern papua new guinea constraints from seismic reflection and well data 1 the d entrecasteaux island dei gneiss domes are fault bounded domes with 2 5 km of relief exposing ultrahigh pressure uhp and high pressure hp metamorphic gneisses and migmatites exhumed in an oligocene miocene arc continent collision and subduction zone subject to late miocene to recent continental extension to study the style of continental extension accompanying exhumation of the dei gneiss domes a grid of 1518 km of 2 d multichannel seismic mcs reflection data and well data is interpreted from the offshore areas surrounding the dei including the trobriand basin and the goodenough basin the offshore study is combined with onshore geologic information to constrain the oligocene to recent tectonic evolution of the basins mcs and well data are consistent with the trobriand basin forming as a forearc basin caused by southward miocene subduction at the trobriand trench at 8 ma the margin transitioned to an extensional tectonic environment since then the trobriand basin has subsided 1 2 5 km with few normal faults deforming the basin fill south of the dei the goodenough rift basin developed after extension began 8 ma with the hanging wall of the north dipping owen stanley normal fault bounding the southern margin of the basin the lack of evidence of upper crustal extension accompanying subsidence in the trobriand and goodenough basins suggests depth dependent lithospheric extension from 8 to 0 ma has accompanied uplift of the dei gneiss domes and supports schematic model of uplift of the dei domes involving vertical exhumation of buoyant postorogenic lower crust far field extension from slab rollback and an inverted two layer crustal density structure

tectonic implications of tomographic images of subducted lithosphere beneath northwestern south america we used seismic tomography to investigate the complex structure of the upper mantle below northwestern south america images of slab structure not delineated by previous seismicity studies help us to refine existing tectonic models of subducted caribbean pacific lithosphere beneath the study area beneath western venezuela and colombia we distinguish two slabs a maracaibo and a redefined bucaramanga slab the maracaibo slab coinciding with most of the bucaramanga slab previously defined by w d pennington dips in a direction of 150 at an angle of 17 to a depth of 275 km and correlates to the subducted late cretaceous oceanic plateau of the caribbean plate farther south a second slab dips at an angle of 50 in a direction of 125 to a depth of at least 500 km and correlates to the subducted oceanic crust of the nazca plate and the downdip extension of the panama island arc we refer to this slab as the redefined bucaramanga slab because it is different from the bucaramanga slab segment defined by pennington the area of the south american plate overriding both slabs is characterized by the absence of an active volcanic arc an anomalously wide topographically uplifted and tectonically active area and the northward escape of the maracaibo block along active strike slip faults in support of earlier studies we attribute this to the underthrusting of the caribbean oceanic plateau our shallowly dipping maracaibo slab along the base of the south american lithosphere and to the recent collision of the panama island arc rafted in on more steeply dipping crust of the nazca plate our redefined bucaramanga slab

flexural and gravity modelling of the merida andes and barinas apure basin western venezuela abstract the kinematic evolution of the barinas apure basin and the southern merida andes from lower miocene to the present is numerically modelled using flexural isostatic theory and geophysical and geological data two published regional transects are used to build up a reference section which is then used to constrain important parameters e g shortenings and sedimentary thicknesses for the flexural modelling to control the location of the main fault system in the flexural model earthquake information is also used the estimated flexural elastic thickness of the south american lithosphere beneath the barinas apure basin and the merida andes range is 25 km the value for the final total shortening is 60 km the flexural isostatic model shows that the andean uplift has caused the south american lithosphere subsidence and the development of the barinas apure basin in addition gravity modelling was used to understand deep crustal features that could not be predicted by flexural theory consequently the best fit flexural model is used to build a gravity model across the merida andes and the barinas apure basin preserving the best controlled structural features from the flexural modelling e g basin wavelength and depth and slightly changing the main bodies density values and deep crustal structures the final gravity model is intended to be representative of the major features affecting the gravity field in the study area the predicted morphology in the lower crustal level of the final gravity model favours the hypothesis of a present delamination or megathrust of the maracaibo crust over the south american shield this process would use the conrad discontinuity as a main detachment surface within an incipient nw dipping continental subduction

rupture across arc segment and plate boundaries in the 1 april 2007 solomons earthquake the largest earthquakes are generated in subduction zones and the earthquake rupture typically extends for hundreds of kilometres along a single subducting plate these ruptures often begin or end at structural boundaries on the overriding plate that are associated with the subduction of prominent bathymetric features of the downgoing plate here we determine uplift and subsidence along shorelines for the 1 april 2007 moment magnitude m w 8 1 earthquake in the western solomon islands using coral microatolls which provide precise measurements of vertical motions in locations where instrumental data are unavailable we demonstrate that the 2007 earthquake ruptured across the subducting simbo ridge transform and thus broke through a triple junction where the australian and woodlark plates subduct beneath the overriding pacific plate previously no known major megathrust rupture has involved two subducting plates we conclude that this event illustrates the uncertainties of predicting the segmentation of subduction zone rupture on the basis of structural discontinuities

microseismicity evidence for subduction of the caribbean plate beneath the south american plate in northwestern venezuela over 1100 microearthquakes with body wave magnitude mb 4 have been located in western venezuela and the southwestern caribbean region since the installation in 1980 of the venezuelan seismological array together with 120 events of mb 4 one of them with surface wave magnitude ms 6 this tectonically complex region is part of the boundary between the caribbean and the south american plates the main seismically active feature inland in western venezuela is the northeast striking 600 km long 100 km wide right lateral strike slip bocono fault zone along the venezuelan andes about 80 of the earthquakes located in the entire region in the period 1980 mid 1995 have occurred on this fault zone at focal depths 20 km microearthquake activity at lower rates also occurs northwest of the venezuelan andes both in the continental and caribbean sea regions part of this activity takes place at depths down to 150 km northwest oriented seismicity depth profiles show the existence of a benioff zone dipping to the southeast beneath northwestern venezuela and northern colombia this indicates the presence of a northeast striking southeast dipping subducted slab of the caribbean plate beneath the south american plate hypocentral locations show that the northeastern end of this subduction occurs northwest of the curacao aruba region in the vicinity of a northwest trending right lateral strike slip fault zone that joins up with the northeastern end of the bocono fault zone this latter place turns out to be the western end of the east west striking san sebastian fault along the venezuelan coast

earthquake synchrony and clustering on fucino faults central italy as revealed from in situ cl 36 exposure dating we recover the holocene earthquake history of seven seismogenic normal faults in the fucino system central italy we collected 800 samples from the well preserved limestone scarps of the faults and modeled their 36cl concentrations to derive their seismic exhumation history we found that 30 large earthquakes broke the faults in synchrony over the last 12 ka the seven faults released strain at the same periods of time 12 9 ka 5 3 ka and 1 5 1 ka on all faults the strain accumulation and release occurred in 3 6 ka supercycles each included a 3 5 ka phase of slow 0 5 2 mm yr strain accumulation in relative quiescence followed by a cluster of three to four large earthquakes or earthquake sequences that released most of the strain in 1 2 ka the large earthquakes repeated every 0 5 0 3 ka during the paroxysmal phases and every 4 3 0 9 ka between those phases earthquakes on the northern faults produced twice larger surface slips 2 m and had larger magnitudes mw 6 2 6 7 than those on the southern faults on most faults the relative strain level was found to control the amount of slip and the time of occurrence of the next large earthquake faults entered a phase of clustered activity once they had reached a specific relative strain threshold the tre monti fault is identified as the most prone to break over the next century our data document earthquake synchrony and clustering at a broader space and time scale than has been reported to date

high precision and high resolution carbonate 230th dating by mc icp ms with sem protocols abstract to facilitate the measurement of u th isotopic compositions suitable for high precision and high resolution 230th dating of coral and speleothem carbonates secondary electron multiplier sem protocol techniques for multicollector inductively coupled plasma mass spectrometry mc icp ms have been developed the instrumental sensitivities are 1 2 with a precision of 1 2 2 for abundance determination of 50 200 fg 234u 1 4 ng 238u or 230th this method features chemistry refinements improvements to procedural and instrumental blanks spectral inference reductions and careful consideration of non linear sem behavior measurement consistency of this mc icp ms combined with previous mass spectrometric results on u th standards and a variety of carbonates demonstrates the validity of the sem protocol method for fossil corals a routine u th isotopic determination at permil level precision requires only 10 50 mg of carbonate as little as 200 mg of young coral with an age of less than 20 yr can be dated with a precision of 0 3 0 8 yr about 20 200 mg speleothem samples with sub ppm to ppm u are required to earn a 5 precision on ages from 5 to 100 kyr requirement of small sample size 10 100s mg carbonate can permit high temporal resolution to date speleothems with slow growth rates i e 1 10 mm kyr this high precision 230th chronology is critical to accurately establish age models date events and splice geochemical proxy time series records from multiple samples in the fields of paleoclimatology and paleoceanography the u th isotopic determination techniques described here can also be applied to different environmental samples such as waters rocks and sediments

development of interconnected talc networks and weakening of continental low angle normal faults fault zones that slip when oriented at large angles to the maximum compressive stress i e weak faults represent a significant mechanical problem here we document fault weakening induced by dissolution of dolomite and subsequent precipitation of calcite abundant talc along a low angle normal fault within the fault core talc forms an interconnected foliated network that deforms by frictional sliding along 50 200 nm thick talc lamellae the low frictional strength of talc combined with dissolution precipitation creep can explain slip on low angle normal faults in addition the stable sliding behavior of talc is consistent with the absence of strong earthquakes along such structures the development of phyllosilicates such as talc by fluid assisted processes within fault zones cutting mg rich carbonate sequences may be widespread leading to profound and long term fault weakness

plate motions in the north andean region repeated geodetic measurements with the global positioning system gps provide direct measurements of displacements due to plate motions and active crustal deformation in central america and northern south america an area of complex interaction of the nazca cocos caribbean and south american plates the displacement rates for the period 1988 1991 obtained from the results of the first three central and south america casa gps campaigns are in general agreement with the predictions of the nuvel 1 plate motion model but there are differences in detail between the observations and the model the nazca north andes convergence rate vector measured by gps is different from the nuvel 1 vector at 95 confidence the difference implies that the north andes are moving northward relative to south america the measured convergence between the caribbean plate and the north andes suggests that the southern margin of the caribbean plate is located in the south caribbean deformed belt the april 1991 costa rica earthquake and the cocos caribbean convergence rate determined by gps suggest the possibility of significant ongoing deformation between central america and the stable interior of the caribbean plate our gps results are consistent with deformation of the overriding plates at the convergent margins of central and south america and confirm that active convergence is occurring around much of the southern margin of the caribbean plate from colombia west to costa rica costa rica and panama are not part of the stable caribbean plate instead the south caribbean deformed belt and the north panama fold belt probably represent the southern margin of the caribbean plate

the 1967 caracas earthquake fault geometry direction of rupture propagation and seismotectonic implications the fault plane orientation of the july 30 1967 caracas earthquake mw 6 6 has been a source of controversy for several years this strike slip event was originally thought to have occurred on an east west oriented fault plane reflecting the relative motion between the caribbean and south american plates more recently however the complex seismic radiation from this event was interpreted as being indicative of a north south striking fault that ruptured along three en echelon segments in this study we synthesize evidence based on the intensity and damage reports the distribution of aftershocks and the results of a joint formal inversion of the p and sh waves and show that these data clearly indicate that the rupture of the 1967 earthquake occurred on an east west trending fault system using a master event technique the largest aftershock which occurred 40 min after the main event is shown to lie 50 km east of the epicenter of the mainshock the epicentral distances of small aftershocks registered in caracas based on the s p arrival time differences and the polarizations of the p waves are also consistent with these events occurring on an east west oriented fault system north of caracas a joint inversion of the teleseismic p and sh waves recorded on long period seismographs of the world wide standardized seismographic network shows that in a time frame of 65 s four distinct bursts of seismic moment release subevents occurred with a total seismic moment of 8 6 1018 n m the first three subevents triggered sequentially from west to east in a direction that is almost identical to the east west trending nodal planes of the source mechanisms the average depth of these three subevents is 14 km the fourth and last identifiable subevent of the sequence shows a reverse faulting mechanism with the nodal planes oriented roughly east west it occurred at a 21 km depth about 50 km to the north of the fault zone defined by the strike slip subevents this fourth subevent appears to reflect compressional deformation of the southern caribbean possibly related to underthrusting along the proposed curacao trench the complexity of the fault system causing the 1967 earthquake suggests that the relative motion along the caribbean south america plate boundary in central venezuela is taken up over a broad highly faulted and highly stressed zone of deformation and not by a simple major throughgoing fault

normal faulting in the upper continental crust observations from regions of active extension abstract observations of present day normal faulting in regions of active continental extension may be helpful when interpreting the geological record in older extensional basins the most obvious manifestation of active extension is normal faulting in earthquakes earthquake foci are mostly confined to the upper seismogenic continental crust whose thickness imposes a scale on the observed deformation large earthquakes move faults with lengths similar or large compared with the thickness of the seismogenic layer large seismogenic normal faults on the continents appear to be restricted to a dip range of around 30 60 dips significantly gentler than 20 have not been observed in fault plane solutions of large earthquakes large seismographic normal faults are approximately planar in cross section and cut through the base of the upper seismogenic layer rotating about a horizontal axis as they move a reasonable estimate of the regional extension can often be made from the dip of the large normal faults and the tilt of the blocks they bound the domino model such faults are rarely continuous for more than 15 20 km but commonly change strike or step in an en echelon fashion this segmentation may occur on a scale controlled by the thickness of the seismogenic layer and is an important influence on sedimentation and drainage these observations are common to large seismogenic normal faults from a variety of tectonic settings and suggest that the kinematics of normal faulting may not be strongly influenced by the forces responsible for the extension which can vary widely in nature and magnitude small earthquakes which move faults that are small compared with the seismogenic layer thickness show no simple pattern and often can be interpreted as internal deformation of blocks bounded by large faults in some places the seismogenic basement faults do not reach the surface but are decoupled from the sedimentary cover by layers of weak lithology faults in the sedimentary cover may be strongly curved in cross section requiring their hangingwalls to deform internally estimating extension from such faults is not straightforward and requires a knowledge of the hangingwall deformation estimates made in this way are often very non unique and prone to large errors we postulate that the thickness of the seismogenic upper crust controls both the maximum length of large normal fault segments along strike and the maximum size of blocks that can rotate coherently about a horizontal axis

subduction of the caribbean plate and basement uplifts in the overriding south american plate the new tectonic interpretations presented in this paper are based on geologic field mapping and gravity data supplemented by well logs seismic profiles and radiometric and earthquake data the present caribbean south american plate boundary is the south caribbean marginal fault where subduction is indicated by folding and thrusting in the deformed belt and a seismic zone that dips 30 to the southeast and terminates 200 km below the maracaibo basin the caribbean south american convergence rate is estimated as 1 9 0 3 cm yr on the basis of the 390 km length of the seismic zone and a thermal equilibration time of 10 m y the caribbean south american convergence has produced a northwest southeast maximum principal stress direction 1 in the overriding south american plate the mean 1 direction for the maracaibo santa marta block is 310 10 based on earthquake focal mechanism determinations and structural and gravity data on the overriding south american plate basement blocks have been uplifted 7 12 km in the last 10 m y to form the venezuelan andes sierra de perija and the colombian santa marta massif crystalline basement of the venezuelan andes has been thrust to the northwest over tertiary sediments on a fault dipping about 25 and extending to the mantle in the sierra de perija mesozoic sediments have been thrust 16 26 km to the northwest over tertiary sandstones along the cerrejon fault a thrust fault dipping 15 10 to the southeast is consistent with field mapping and gravity and density data the santa marta massif has been uplifted 12 km in the last 10 m y by northwest thrusting over sediments the basement block overthrusts of the perijas venezuelan andes and the santa marta massif are pliocene pleistocene analogs for laramide orogenic structures in the middle and southern rocky mountains of the united states the nonmagmatic basement block uplifts along low angle thrust faults reveal horizontal compression in the overriding plate over 500 km from the convergent margin present day east northeast west southwest 080 compression is indicated by earthquake focal mechanisms and strike slip motion on the bocono fault these earthquakes are intraplate deformation associated with east west 080 nazca south american convergence

continental breakup and uhp rock exhumation in action gps results from the woodlark rift papua new guinea we show results from a network of campaign global positioning system gps sites in the woodlark rift southeastern papua new guinea in a transition from seafloor spreading to continental rifting gps velocities indicate anticlockwise rotation at 2 2 7 myr relative to australia of crustal blocks north of the rift producing 10 15 mm yr of extension in the continental rift increasing to 20 40 mm yr of seafloor spreading at the woodlark spreading center extension in the continental rift is distributed among multiple structures these data demonstrate that low angle normal faults in the continents such as the mai iu fault can slip at high rates nearing 10 mm yr extensional deformation observed in the d entrecasteaux islands the site of the world s only actively exhuming ultra high pressure uhp rock terrane supports the idea that extensional processes play a critical role in uhp rock exhumation gps data do not require significant interseismic coupling on faults in the region suggesting that much of the deformation may be aseismic westward transfer of deformation from the woodlark spreading center to the main plate boundary fault in the continental rift the mai iu fault is accommodated by clockwise rotation of a tectonic block beneath goodenough bay and by dextral strike slip on transfer faults within and surrounding normanby island contemporary extension rates in the woodlark spreading center are 30 50 slower than those from seafloor spreading derived magnetic anomalies the 0 5 ma to present seafloor spreading estimates for the woodlark basin may be overestimated and a reevaluation of these data in the context of the gps rates is warranted

man made changes of seismicity rates seismicity catalogs contain important information about processes which occur in seismically active regions of the earth many authors have examined these catalogs for patterns and variations in patterns which might reflect changes in these processes we have found that these catalogs include a complex mixture of real and man made changes one must identify and account for the man made changes before the real ones can be identified and understood many man made changes in seismicity catalogs are manifested as changes in seismicity rates and can therefore be identified through careful examination of these rates obvious effects include increases or decreases in the detection and reporting of smaller events which accompany the installation or closure of seismic stations these types of changes can be recognized by examining the distribution of seismicity rate changes in the magnitude domain they are characterized by increases or decreases in the number of small events in the catalog at times when the number of larger events remains constant systematic changes in the magnitudes assigned to events can also be identified by examining seismicity rates because they cause apparent changes in rates of data sets with magnitude cutoffs the sign of the apparent rate change depends on the sign of the magnitude change and the type of cutoff used the effects of detection changes can be easily remedied by using a magnitude cutoff which eliminates the smaller events from consideration the techniques we have developed allow one to determine the optimum cutoff magnitude corrections are necessary for remedying the effects of systematic changes in magnitude estimates these corrections can be determined through modeling of observed rate changes caused by these shifts man made changes are present in all seismicity catalogs whether local regional or teleseismic they must be accounted for if these catalogs are to provide meaningful information on real process changes in the earth

tectonics of the new guinea region the new guinea region evolved within the obliquely and rapidly converging australian and pacific plate boundary zone it is arguably one of the most tectonically complex regions of the world and its geodynamic evolution involved microplate formation and rotation lithospheric rupture to form ocean basins arc continent collision subduction polarity reversal collisional orogenesis ophiolite obduction and exhumation of ultra high pressure metamorphic rocks we describe the major onshore and offshore tectonic and geologic components including plate boundaries seismicity faults and magmatism and we integrate these with emerging ideas about mantle dynamics to evaluate the cenozoic tectonic evolution of new guinea future research opportunities to resolve the mantle structure beneath new guinea will enable mantle dynamics to be linked to lithospheric and surface processes virtually all plate tectonic and mantle processes have been active in the new guinea region throughout the cenozoic and as such

shallow dips of normal faults during rapid extension earthquakes in the woodlark d entrecasteaux rift system papua new guinea earthquakes in the woodlark d entrecasteaux rift system reveal changes in fault geometry along strike as extension progresses from continental to oceanic modes in one segment evidence is found for earthquakes on shallow dipping normal faults to examine these trends locations of 147 earthquakes are redetermined and focal mechanisms for 21 events are derived from waveform inversion where seafloor spreading occurs east of 152 e most earthquakes illuminate a ne sw strike slip fault north of the oceanic rift system along the woodlark rise elsewhere earthquakes occur along a well defined rift zone and focal mechanisms reveal normal faulting with north south t axes and depths less than 10 km most normal faulting earthquakes show high angle fault planes dips 40 50 but several are consistent with slip on moderately low angle planes dipping 15 35 these low angle nodal planes are only found in a region between 150 5 e and 152 5 e where the transition occurs from seafloor spreading to continental rifting a seismic reflection profile near 151 e reveals a major normal fault that dips to the north at 25 35 just landward of the oceanic rift tip that fault and parallel structures are excellent candidates for the fault that slipped in at least two of the earthquakes studied hence normal faults dipping 30 are likely to be seismically active in at least one part of this rift system in this region just landward from the rift tip the highest extension rates are expected 40 mm yr and strain appears localized on a single system of normal faults because shallow dips for normal faults are seen near the rift tip but not elsewhere along strike high strain rates may be an important factor in allowing normal faults to slip at unusually shallow dips perhaps high shear traction at the base of the upper crust localized by nearby spreading sufficiently alters the local stress regime

frictional properties of a low angle normal fault under in situ conditions thermally activated velocity weakening the zuccale fault is a regional low angle normal fault exposed on the isle of elba in central italy that accommodated a total shear displacement of 6 8 km the fault zone structure and fault rocks formed at 8 km crustal depth the present day fault structure is the final product of several deformation processes superposed during the fault history in this study we report results from a series of rotary shear experiments performed on 1 mm thick powdered gouges made from several fault rock types obtained from the zuccale fault the tests were done under conditions ranging from room temperature to in situ conditions i e at temperatures up to 300 c applied normal stresses up to 150 mpa and fluid saturated the ratio of fluid pressure to normal stress was held constant at either 0 4 or 0 8 to simulate an overpressurized fault the samples were sheared at a constant sliding velocity of 10 m s for at least 5 mm after which a velocity stepping sequence from 1 to 300 m s was started to determine the velocity dependence of friction this can be represented by the rate and state parameter a b which was determined by an inversion of the data to the rate and state equations friction of the various fault rocks varies between 0 3 and 0 8 similar to values obtained in previous studies and decreases with increasing phyllosilicate content friction decreases mildly with temperature whereas normal stress and fluid pressure do not affect friction values systematically all samples exhibited velocity strengthening conditionally stable behavior under room temperature conditions and a b increased with increasing sliding velocity in contrast velocity weakening accompanied by stick slips was observed for the strongest samples at 300 c and sliding velocities below 10 m s an increase in fluid pressure under these conditions led to a further reduction in a b for all samples so that they exhibited unstable stick slip behavior at low sliding velocity the results suggest that phyllosilicate bearing fault rocks can deform by stable aseismic creep at low resolved shear stress and low shear rate conditions an increase in fluid pressure or loading of stronger portions could lead to a runaway instability the runaway instability might be limited in size because of 1 the fault heterogeneity 2 the observed strengthening at higher sliding velocities and 3 a co seismic drop in pore fluid pressure

creep and locking of a low angle normal fault insights from the altotiberina fault in the northern apennines italy while low angle normal faults have been recognized worldwide from geological studies whether these structures are active or capable of generating big earthquakes is still debated we provide new constraints on the role and modes of the altotiberina fault atf in accommodating extension in the northern apennines we model gps velocities to study block kinematics faults slip rates and interseismic coupling of the atf which is active and accounts with its antithetic fault for a large part of the observed chain normal 3 mm yr tectonic extension a wide portion of the atf creeps at the long term slip rate 1 7 0 3 mm yr but the shallow locked portions are compatible with m 6 5 earthquakes we suggest that positive stress accumulation due to atf creep is most likely released by more favorable oriented splay faults whose rupture may propagate downdip along low angle normal fault surface and reduce the probability of occurrence of a seismic rupture of the shallower locked portion

southeast papuan crustal tectonics imaging extension and buoyancy of an active rift southeast papua hosts the world s youngest ultra high pressure uhp metamorphic rocks these rocks are found in an extensional setting in metamorphic core complexes competing theories of extensional shear zones or diapiric upwelling have been suggested as driving their exhumation to test these theories we analyze the cdpapua temporary array of 31 land and 8 seafloor broadband seismographs seismicity shows that deformation is being actively accommodated on the core complex bounding faults offset by transfer structures in a manner consistent with overall north south extension rather than radial deformation rayleigh wave dispersion curves are jointly inverted with receiver functions for crustal velocity structure they show crustal thinning beneath the core complexes of 30 50 and very low shear velocities at all depths beneath the core complexes on the rift flanks velocities resemble those of normal continents and increase steadily with depth there is no evidence for velocity inversions that would indicate that a major density inversion exists to drive crustal diapirs also low density melt seems minor within the crust together with the extension patterns apparent in seismicity these data favor an extensional origin for the core complexes and limit the role of diapirism as a secondary exhumation mechanism although deeper mantle diapirs may be undetected a small number of intermediate depth earthquakes up to 120 km deep are identified for the first time just northeast of the d entrecasteaux islands they occur at depths similar to those recorded by uhp rocks and similar temperatures indicating that the modern seismicity occurs at the setting that generates uhp metamorphism

structure of the merida andes venezuela relations with the south america caribbean geodynamic interaction for over 50 years several models based on diverse geologic concepts and variable quality of data have been proposed to explain the major structure and history of the merida andes ma in western venezuela lately this chain growth and associated flexural basins deepening have been related to incipient type a subductions of either polarity accounting for the across chain asymmetry however these recent models have not well integrated the present tectonically active setting driven by neighboring major plate interactions at present this chain exhibits ongoing strain partitioning where cumulative right lateral slip along chain axis is as much as half of or about the same as the transverse shortening since late miocene thus implying that the nne directed maracaibo block extrusion with respect to the south america sa plate is not a secondary feature consequently this paper discusses some limitations exhibited by the se directed continental subduction models maracaibo crust underthrusting the merida andes in the light of available geological and geophysical data besides it is herein proposed that the merida andes structuration is related to a nw directed gently dipping incipient type a subduction where chain growth and evolution are similar to those of a sedimentary accretionary wedge i e barbados but at crustal scale and with ongoing strain partitioning this continental subduction is the se portion of a major orogenic float that also comprises the perija range and the santa marta block d 2002 elsevier science b v all rights reserved

evaluating upper versus lower crustal extension through structural reconstructions and subsidence analysis of basins adjacent to the d entrecasteaux islands eastern papua new guinea 1 the d entrecasteaux island dei gneiss domes are fault bounded domes with 2 5 km of relief exposing ultrahigh pressure uhp and high pressure hp metamorphic gneisses and migmatites exhumed in an oligocene miocene arc continent collision and subduction zone subject to late miocene to recent continental extension multichannel seismic reflection data and well data show the trobriand basin formed as a fore arc basin caused by southward miocene subduction at the trobriand trench subduction slowed at 8 ma as the margin transitioned to an extensional tectonic environment since then the trobriand basin has subsided 1 2 5 km as a broad sag basin with few normal faults deforming the basin fill south of the dei the goodenough rift basin developed after extension began 8 ma as the hanging wall of the north dipping owen stanley normal fault that bounds the basin s southern margin the lack of upper crustal extension accompanying subsidence in the trobriand and goodenough basins suggests depth dependent lithospheric extension since 8 ma has accompanied uplift of the dei gneiss domes structural reconstructions of seismic profiles show 2 3 13 4 km of basin extension in the upper crust while syn rift basin subsidence values indicate at least 20 7 23 6 km of extension occurred in the entire crust since 8 ma results indicating thinning is preferentially accommodated in the lower crust surrounding the dei are used to constrain a schematic model of uplift of the dei domes involving vertical exhumation of buoyant postorogenic lower crust far field extension from slab rollback and an inverted two layer crustal density structure

neotectonics of the caribbean a compilation of earthquake focal mechanism solutions together with the interpretation of inclined intermediate depth seismic zones neogene strike slip faults fault controlled basins and volcanism is used to test three existing models for the neotectonic i e neogene or post oligocene development of the caribbean the current models although based on different hypotheses using different data sets are similar and suggest varying degrees of convergence between the caribbean and either north or south american plates or convergence between the caribbean plate and continental fragments of the northwestern corner of south america using model 1 previous workers have interpreted the disposition of fracture zones and magnetic anomalies in the central and south atlantic as indicating that the caribbean plate is being compressed by north south convergence between the americas with the result that it is being driven eastward relative to the americas along strike slip faults while producing east west trending subductionlike convergent zones along northern and southern plate margins using model 2 previous workers have interpreted the azimuths of active strike slip faults and focal mechanism solutions and closure of three plate velocity circuits as indicating that the south american plate converges in a west northwesterly direction on the southern margin of the caribbean plate and has produced compressional features while strike slip motion dominates along the northern boundary using model 3 previous workers have interpreted focal mechanism solutions and the pattern of strike faulting and basin development in northwestern south america to suggest right lateral displacement of the northwestern corner of south america relative to the craton with convergence and compression in the south central caribbean our integration of geologic information on faults basins and volcanism considerably extends the limited record of plate interactions known from historical reports of the past few hundred years and recorded earthquakes and suggests that 1 relative motion between the north american and caribbean plates is dominantly east west strike slip motion although areas of complex internal deformation associated with restraining bend or compressional strike slip fault segments have been identified in jamaica hispaniola and central america 2 relative motion between the south american and caribbean plates is dominantly east west strike slip motion although an area of restraining bend deformation has been identified in the araya peninsula of venezuela 3 relative motion between a triangular continental fragment of northwestern south america the maracaibo block ranges from northwest to northeast along the south central margin of the caribbean and appears to be driven mostly by the late miocene pliocene collision of the panama arc although additional driving forces may include the collision of the carnegie ridge and or oblique subduction to the northeast in ecuador and 4 relative motion internal deformation may be occurring at a very slow rate between fault bounded blocks within the caribbean plate as a result of the convergence of the maracaibo block and or eastward motion of the caribbean plate relative to a continental promontory of north america in northern central america because the caribbean margins are composed mainly of anisotropic easily deformed continental and arc lithosphere plate boundaries are zones of diffuse deformation and it is difficult to assess the degree of convergence between the americas although data from the caribbean itself suggest that it may be smaller than the amount inferred by some workers using model 1 fracture zone and magnetic anomalies in the atlantic some of the compressional features cited as evidence for the relative convergence of the americas may be instead related to the convergence between the maracaibo block relative to the southern margin of the caribbean many aspects of caribbean neotectonics remain less than completely understood but more integration of seismic and geologic studies particularly exploratory fault trenching of onshore faults and seismic profiling of offshore faults should lead to rapid advances

gps and seismological constraints on active tectonics and arc continent collision in papua new guinea implications for mechanics of microplate rotations in a plate boundary zone 1 the island of new guinea is located within the deforming zone between the pacific and australian plates that converge obliquely at 110 mm yr new guinea has been fragmented into a complex array of microplates some of which rotate rapidly about nearby vertical axes we present velocities from a network of 38 global positioning system gps sites spanning much of the nation of papua new guinea png the gps derived velocities are used to explain the kinematics of major tectonic blocks in the region and the nature of strain accumulation on major faults in png we simultaneously invert gps velocities earthquake slip vectors on faults and transform orientations in the woodlark basin for the poles of rotation of the tectonic blocks and the degree of elastic strain accumulation on faults in the region the data are best explained by six distinct tectonic blocks the australian pacific south bismarck north bismarck and woodlark plates and a previously unrecognized new guinea highlands block significant portions of the ramu markham fault appear to be locked which has implications for seismic hazard determination in the markham valley region we also propose that rapid clockwise rotation of the south bismarck plate is controlled by edge forces initiated by the collision between the finisterre arc and the new guinea highlands

orogenic float of the venezuelan andes abstract the venezuelan or merida andes are a ne trending intracontinental orogen that started to rise from the middle miocene due to the e w far field convergence between the maracaibo block to the northwest and the guyana shield to the southeast oblique convergence is responsible for strain partitioning with thrusting along both foreland basins and right lateral strike slip faulting along the ne sw bocono fault cutting the venezuelan andes along strike the central part of the belt is also cut by the n s left lateral strike slip valera fault that connects the bocono fault both faults bounding the trujillo block that escapes towards the nne even though the regional geology of belt is well known its structure at depth remains a matter of debate our work based on the integration of geological and geophysical data aims to better constrain the deep geometry of faults and the tectonic evolution of the mountain belt we used the orogenic float model to construct two nw se trans andean crustal scale balanced sections the late neogene quaternary shortening varies from 40 km in the south to 30 km in the north across the trujillo block indicating that a quarter of the deformation seems to be absorbed by the tectonic escape process more importantly a major reorganization in the crust took place in the early pliocene it is characterized by the imbrication of the maracaibo crust into the guyana crust this resulted in the subduction of the guyana lower crust and the formation of a nw vergent basement thrust propagating upwards and surfacing along the las virtudes thrust rapid uplift of the northern flank of the belt subsequently occurred together with massive deposition of the plio quaternary coarse grained betijoque formation in the northwestern foreland basin

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

present activity and seismogenic potential of a low angle normal fault system citta di castello italy constraints from surface geology seismic reflection data and seismicity abstract we present new constraints on an active low angle normal fault system in the citta di castello sansepolcro basin csb of the northern apennines of italy new field data from the geological survey of the carta geologica d italia carg project define the surface geometry of the normal fault system and lead to an interpretation of the crop 03 deep crust seismic reflection profile castiglion fiorentino urbania segment with particular attention paid to the geometry of the plio quaternary extensional structures surface and sub surface geological data are integrated with instrumental and historical seismicity in order to define the seismotectonics of the area low angle east dipping reflectors are the seismic expression of the well known altotiberina fault af a regional extensional detachment on which both east and west dipping high angle faults bounding the csb sole out the af breakaway zone is located 10 km west of the csb within the extensional allochthon synthetic east dipping planes prevail displacement along the af is 4 5 km which agrees with the cumulative offset due to its synthetic splays the evolution of the csb has mainly been controlled by the east dipping fault system at least since early pleistocene time this system is still active and responsible for the seismicity of the area a low level of seismic activity was recorded instrumentally within the csb but several damaging earthquakes have occurred in historical times the instrumental seismicity and the intensity data points of the largest historical earthquakes 5 events with maximum mcs intensity of ix to ix x allow us to propose two main seismogenic structures the monte santa maria tiberina mmax 5 9 and citta di castello mmax up to 6 5 normal faults both are synthetic splays of the af detachment dipping to the ne at moderate 45 50 to low 25 30 angles and cutting the upper crust up to the surface this study suggests that low angle normal faults at least with dips of 25 30 may be seismogenic

a mechanism to thin the continental lithosphere at magma poor margins crustal thinning where continental plates break apart can be accomplished by a system of conjugate concave downward faults instead of multiple normal faults through exhumation of mid crustal and mantle material where continental plates break apart slip along multiple normal faults provides the required space for the earth s crust to thin and subside1 after initial rifting however the displacement on normal faults observed at the sea floor seems not to match the inferred extension2 here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust 10 km by exhumation of mid crustal and mantle material this transitional crust is capped by sub horizontal detachment surfaces over distances exceeding 100 km with little visible deformation our rift model is based on numerical experiments constrained by geological and geophysical observations from the alpine tethys and iberia newfoundland margins3 4 5 6 7 8 9 furthermore we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust the presence of such lower crustal gabbros is well constrained for the alpine tethys system4 9 initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution this is achieved in our models by the formation of weak ductile shear zones

earthquakes and tsunamis caused by low angle normal faulting in the banda sea indonesia as the world s largest archipelagic country in earth s most active tectonic region indonesia faces a substantial earthquake and tsunami threat understanding this threat is a challenge because of the complex tectonic environment the paucity of observed data and the limited historical record here we combine information from recent studies of the geology of indonesia s banda sea with global positioning system observations of crustal motion and an analysis of historical large earthquakes and tsunamis there we show that past destructive earthquakes were not caused by the supposed megathrust of the banda outer arc as previously thought but are due to a vast submarine normal fault system recently discovered along the banda inner arc instead of being generated by coseismic seafloor displacement we find the tsunamis were more likely caused by earthquake triggered submarine slumping along the fault s massive scarp the weber deep this would make the banda detachment representative not only as a modern analogue for terranes hyper extended by slab rollback but also for the generation of earthquakes and tsunamis by a submarine extensional fault system our findings suggest that low angle normal faults in the banda sea generate large earthquakes which in turn can generate tsunamis due to earthquake triggered slumping low angle normal faults in the banda sea have caused large earthquakes that indirectly generated tsunamis due to earthquake triggered submarine slumping according to an analysis of historical earthquake and tsunami events and gps observations

low angle normal faults and seismicity a review although large low angle normal faults in the continental crust are widely recognized doubts persist that they either initiate or slip at shallow dips 30 because 1 global compilations of normal fault focal mechanisms show only a small fraction of events with either nodal plane dipping less than 30 and 2 andersonian fault mechanics predict that normal faults dipping less than 30 cannot slip geological reconstructions thermochronology paleomagnetic studies and seismic reflection profiles mainly published in the last 5 years reinforce the view that active low angle normal faulting in the brittle crust is widespread underscoring the paradox of the seismicity data for dip slip faults large enough to break the entire brittle layer during earthquakes m w 6 5 consideration of their surface area and efficiency in accommodating extension as a function of dip suggests average recurrence intervals of earthquakes r tan assuming stress drop rigidity modulus and thickness of the seismogenic layer do not vary systematically with dip if the global distribution of fault dip normalized to total fault length is uniform the global recurrence of earthquakes as a function of dip is shown to be r tan sin this relationship predicts that the frequency of earthquakes with nodal planes dipping between 30 and 60 will exceed those with planes shallower than 30 by a factor of 10 in good agreement with continental seismicity assuming major normal faults dipping more than 60 are relatively uncommon revision of andersonian fault mechanics to include rotation of the stress axes with depth perhaps as a result of deep crustal shear against the brittle layer would explain both the common occurrence of low angle faults and the lack of large faults dipping more than 60 if correct this resolution of the paradox may indicate significant seismic hazard from large low angle normal faults

global trench migration velocities and slab migration induced upper mantle volume fluxes constraints to find an earth reference frame based on minimizing viscous dissipation sincetheadventofplatetectonicsdifferentglobalreferenceframeshavebeenusedtodescribethemotionofplatesandtrenches thedifferencein plate motion and trench migration between different reference frames can be substantial up to 4 cm yr this study presents an overview of trench migration velocities for all the mature and incipient subduction zones on earth as calculated in eight different global reference frames calculations show that irrespective of the reference frame 1 trench retreat always dominates over trench advance with 62 78 of the 244 trench segments retreating 2 the mean and median trench velocity are always positive retreating and within the range 1 3 1 5 cm yr and 0 9 1 3 cm yr respectively 3 rapid trench retreat is only observed close to lateral slab edges b1500 km and 4 trench retreat is always slow far from slab edges n2000km thesecalculationsarepredictedbygeodynamicmodelswithavaryingslabwidth inwhichplatemotion trenchmotionandmantleflow result from subduction ofdense slabs suggesting that trenchmotion is indeedprimarily drivenby slab buoyancyforces andthat proximity to alateral slab edge exerts a dominant control on the trench migration velocity despite these four general conclusions significant differences in velocities between such reference frames remain it is therefore important to determine which reference frame most likely describes the true absolute velocities togetanunderstandingoftheforcesdrivingplatetectonicsandmantleconvection itishereproposedthat basedonfluiddynamicconsiderationsand predictions from geodynamic modelling the best candidate is the one which optimises the number of trench segments that retreat minimizes the trench perpendicular trench migration velocity vt in the centre of wide n4000 km subduction zones maximizes the number of retreating trench segments located within 2000 km of the closest lateral slab edge minimizes the average of the absolute of the trench perpendicular trench migration velocity vt forallsubductionzonesonearth andminimizestheglobaluppermantletoroidalvolumeflux to thatresultsfromtrenchmigration and associated lateral slab migration i e slab rollback or slab roll forward calculations show that these conditions are best met in one particular indo atlantic hotspot reference frame where 75 of the subduction zones retreat vt in the centre of wide subduction zones ranges between 3 5 and 1 8 cm yr 83 of the trench segments located within 2000 km of the closest lateral slab edge retreat the average of vt is 2 1 cm yr and to 456 km 3 yr lower limit and 539 km 3 yr upper limit inclusion of all the incipient subduction zones on earth results in slightly greater fluxes of 465 km 3 yr lower limit and 569 km 3 yr upper limit it is also found that this reference frame is close to minimizing the total sub lithospheric uppermantlevolumeflux k associatedwithmotionofcontinentalkeelslocatedbelowthemajorcratons itisstressed however that kisanorder of magnitude smaller than to and thus of subordinate importance in conclusion the indo atlantic hotspot reference frame appears preferable for calculating plate velocities and plate boundary velocities

holocene sea level highstand and its implications for the vertical stability of panglao island southwest bohol philippines abstract emerged tidal notches in panglao island southwest bohol central philippines were used to investigate relative sea level changes during the holocene the elevation and profile of a laterally continuous notch were surveyed at 22 sites around panglao island the tidal notch documents higher than present mean sea level pmsl consistent with regional field observations and predictions for mid holocene time the reconstructed paleo shoreline elevations range from 0 to 1 6 m above pmsl exhibiting a general increase to the northeast the more protected part of the coast this lateral variation in the holocene paleo mean sea level msl along with changes in notch depth and profile suggests tilting of the island best explained by differential land movement last interglacial msl estimates by previous workers also show tilting to the southwest moreover the holocene notch data suggest segmentation of the island into several blocks that tilt consistently to the southwest suggesting the presence of faults lineations in aerial photographs trending northeast southwest confirm the presence of these faults these data indicate differential land movements during the holocene and late pleistocene calculated rates of uplift are 0 3 and 0 06 mm yr for the holocene and for the late pleistocene respectively these values relative to the high uplift rates from tectonically active regions imply that panglao is relatively vertically stable after accounting for tectonic tilting mid holocene sea level is estimated at approximately 0 3 0 6 m

modern vertical deformation above the sumatran subduction zone paleogeodetic insights from coral microatolls coral microatolls from the coast and outer arc islands of western sumatra retain a stratigraphic and morphologic record of relative sea level change which is due in large part to vertical tectonic deformation above the sumatran subduction zone low water levels whose fluctuations produce measurable changes in coral morphology limit the upward growth of the microatolls annual rings derived from seasonal variations in coral density serve as an internal chronometer of coral growth the microatolls act as natural long term tide gauges recording sea level variations on time scales of decades field observations and stratigraphic analysis of seven microatolls five from the outer arc islands and two from the mainland coast indicate that the mentawai islands have been submerging at rates of 4 10 mm yr over the last four or five decades while the mainland has remained relatively stable the presence of fossil microatolls up to several thousand years old in the intertidal zone indicates that little permanent vertical deformation has occurred over that time thus most of the strain accumulated in the past few decades represents interseismic deformation that is recovered during earthquakes elastic dislocation models using these submergence data suggest that elastic strain is being accumulated in the interseismic period and that the subduction zone in this region is fully coupled

analysis of partially emerged corals and reef terraces in the central vanuatu arc comparison of contemporary coseismic and nonseismic with quaternary vertical movements in the central vanuatu arc living and recently deceased reef corals act as natural tide gauges which have allowed us to map vertical tectonic deformation patterns as corals grow the density of the aragonite coral skeletons varies on an annual cycle producing annual growth bands similar to tree rings using coral growth bands we can determine the year coral surfaces died due to emergence we interpret four major coral emergence events as coseismic uplifts that occurred near the epicenters and times of large shallow earthquakes on january 5 1946 ms 7 3 august 11 1965 ms 7 5 october 27 1971 ms 7 1 and december 29 1973 ms 7 5 the 1965 and 1973 events caused maximum uplifts of 120 and 60 cm respectively in the frontal arc also related to these events are uplifts of 10 cm and 6 cm in the back arc on pentecost and maewo islands which lie east of the volcanic chain and the primary forearc zones of uplift and subsidence similar secondary zones of uplift occurred with the great 1960 chile and 1964 alaska earthquakes the amplitude of these secondary uplifts is significantly larger than that predicted by models having a single fault in an elastic half space however the amount of secondary uplift is comparable to that predicted if the fault occurs in a plate of constant thickness overlying a viscoelastic half space at various places in 1957 1969 1970 1977 and 1978 1981 there was about 5 10 cm of emergence not associated with major earthquakes which may indicate nonseismic tectonic uplift however oceanographically lowered sea levels as in el ninos may have determined the times when corals died and recorded these events nevertheless the accumulation of emergence its persistence the limited geographic extent of each event and occurrence in areas of rapid holocene uplift suggest that the causes of the uplifts are tectonic these events suggest that in some areas a third or more of the total accumulated uplift in central vanuatu takes place as aseismic motion however in some areas we find only coseismic emergence in central vanuatu contemporary coseismic vertical deformation holocene uplift and topography have remarkably similar patterns this suggests that the mechanisms and processes causing vertical deformation have varied little over the last 106years apparently the topography structure and seismotectonics are controlled by the subduction of the d entrecasteaux ridge a major bathymetric feature underthrusting this part of the arc the influence of this ridge may have been especially extensive because it migrates very slowly along the arc trend and thus it interacts for a long time with a single portion of the arc system our previous studies of reef terraces indicated the existence of at least four seismotectonic arc segments or blocks along the santo malekula interval of the arc and our present results further support this conclusion each block has uplifted at different times by different amounts at different rates and tilted in a different direction boundaries between the north santo and the south santo segment and between the north malekula and the south malekula segment correlate with the north and south flanks of the d entrecasteaux ridge as does the absence of a physiographic trench west of santo

heterogeneities of the instrumental seismicity catalog 1904 1980 for strong shallow earthquakes we analyze the world wide consistency of teleseismic reporting completeness of the seismicity record and homogeneity of magnitude determination for strong shallow earthquakes magnitude m 6 depth h 70 km in the period 1904 to 1980 for earthquakes with magnitude m 7 we used the catalog given by abe 1981 which lists the surface wave magnitude ms of every reported large event that occurred between 1904 and 1980 according to the original formulation of gutenberg 1945 under the postulate that the rate of earthquake occurrence for the entire world as a whole is consistent on a time scale of decades and that since the installation of the world wide standardized seismograph network in the early 1960s the earthquake catalog for strong m 6 shocks is complete and the seismicity rates are typical of all periods in the century we find that for instrumental related reasons the ms of large events prior to 1908 have been systemically overestimated by at least 0 5 magnitude unit and the ms of events in the period 1908 to 1948 have been consistently overestimated by 0 2 unit relative to the ms assigned to shocks occurring after 1948 when these corrections are taken into account the catalog of events with ms 7 0 is shown to be largely complete i e nearly all the large shallow shocks which occurred in the earth are listed for most of the world since early in the century for earthquakes as the magnitude 6 level we used the international seismological center isc tape and regional catalogues of earthquakes this file compiles the earthquake data given by gutenberg and richter 1954 for the first half of the century and data from various international agencies after 1945 we find a highly incomplete seismicity catalog at this magnitude level for the first half of the century mainly due to numerous increases and decreased in the earthquake detection and reporting capabilities a more complete but extremely inhomogeneous catalog for the period 1950 to 1963 due to the use of different formulations and criteria to calculate and report the parameter magnitude by the various seismological agencies and a largely complete and homogeneous in ms record since 1964

the josephine ophiolite northwestern california the 157 m y old josephine ophiolite is a complete ophiolite consisting of the following units harzburgite tectonite cumulate ultramafics cumulate gabbro interlayered with cumulate ultramafics cumulate gabbro high level gabbro and diorite a 1 5 km thick sheeted dike complex and 0 4 km of pillow lava and breccia the ophiolite is conformably overlain by the galice formation consisting of a thin siliceous pelagic sequence overlain by a thick sequence of turbidites having both volcanic arc and continental margin sources the ophiolite and overlying galice formation were intruded by numerous 150 to 151 m y old calc alkaline dikes and sills followed shortly by deformation and regional metamorphism under conditions of prehnite pumpellyite to lower greenschist facies the josephine peridotite makes up the base of the ophiolite and covers 800 km2 it is an alpine type peridotite that represents upper mantle material and the residue of partial melting ultramafic rocks overlying the josephine peridotite are olivine spinel clinopyroxene cumulates that grade upward into clinopyroxene and orthopyroxene olivine plagioclase cumulates and finally to clinopyroxene plagioclase cumulates high level gabbros and diorites are characterized by highly variable textures complex intrusive relationships strongly zoned plagioclase and abundant mafic dikes plagiogranites occur at this level as dikes small intrusions and screens between mafic dikes but they are rare the sheeted dike complex consists of mafic dikes averaging 0 6 m in thickness chilled margins are locally well preserved and many of the dikes have been split and intruded by younger dikes resulting in half and marginless dikes the dike complex is composed of 100 sheeted dikes near the center but contains gabbro screens near the base and pillow lava and breccia screens in its upper parts the dikes are inclined at 40 to bedding after correcting for folding and are inferred to have been rotated shortly after formation the extrusive sequence consists predominantly of pillow lavas but contains abundant pillow breccias and some massive lavas some of the breccias formed as talus deposits whereas others are graded and matrix supported indicating transport and redeposition possibly in an area of rugged submarine topography the dikes and lavas have geochemical affinities to island arc tholeiites but are in some aspects transitional to mid ocean ridge basalts the dikes and lavas are cogenetic with the underlying plutonic sequence as indicated by similarity in crystallization sequence contradictory intrusive relationships between sheeted dikes and high level plutonic rocks and lack of diabase dikes in the lowermost cumulates and harzburgite the upper portions of the ophiolite were hydrothermally metamorphosed at the spreading center resulting in downward changes in mineralogy texture and 18o in addition metalliferous sediments and massive sulfides that occur overlying and within the pillow lavas probably were formed by precipitation from thermal springs a geothermal gradient of 250 350 c km is inferred from mineral assemblages on the basis primarily of its regional setting the ophiolite is inferred to have formed in a back arc basin in particular it is within a thrust sheet sandwiched between a coeval magmatic arc complex and a slightly older arc complex remnant arc this interpretation is consistent with abundant volcanic arc detritus in graywackes overlying the ophiolite the island arc magmatic affinities of the lavas and dikes may be the result of formation of the ophiolite during the earliest stages of back arc spreading

macroseismic interpretation of the 1812 earthquakes in venezuela using intensity uncertainties and a priori fault strike information we applied the bakun and wentworth 1997 method to the 26 march 1812 venezuelan earthquake previous studies have shown that it had at least two large subevents one associated with the bocono fault and an intensity center between barquisimeto and san felipe the other associated with the san sebastian fault near caracas we used the method with two modifications first we introduced intensity intervals to account for the uncertainties associated with historical information second we incorporated a correction that considers the fault s strike but not its actual position these modifications were tested with three earthquakes the 1989 loma prieta a 1980 m w 5 2 event in tachira venezuela and the 1967 earthquake in caracas m w 6 6 the third one was important for the interpretation of the 1812 caracas subevent for the barquisimeto san felipe subevent we obtained m wi 7 4 0 35 and an intensity center 10 20 n 69 95 w that lies between barquisimeto and san felipe our preferred rupture length was between 90 and 100 km the caracas subevent had m wi 7 1 0 33 with an intensity center 10 60 n 67 10 w close to the venezuelan north coast the expected rupture length was on the order of 70 km

intermediate depth seismicity in northern colombia and western venezuela and its relationship to caribbean plate subduction the recent intermediate depth seismicity in northern colombia and western venezuela was analyzed to understand its origin and its presumed relationship to a subducted lithospheric slab in northwestern south america the area included in this study is located to the north and east of the bucaramanga nest which is a particular region beneath bucaramanga northern colombia that presents a high concentration of intermediate depth earthquakes to the north of the nest the seismicity of the area is sparse and most of the events are of low magnitude mb 5 l thus only 23 earthquakes were large enough to be investigated using teleseismic data the focal parameters of the two largest events mb 5 4 were obtained from the formal inversion of long period body waves recorded at teleseismic distances the focal mechanisms of 10 more events were determined from first motion data in total the focal mechanisms of 12 events were determined from both the inversion of p and s h waveforms and the polarities of first arrivals for the smaller earthquakes the focal depths were calculated by measuring the observed pp p interval time and comparing it to the theoretical travel time tables the isodepth curves reflect a slab striking in a nne ssw direction and dipping approximately at 25 32 to the southeast this observation is corroborated by the direction and plunge of the t axes of the focal mechanisms which are generally parallel to the gradient of the slab defined by the spatial distribution of hypocenters these results indicate that the intermediate depth earthquakes in western venezuela and northern colombia are apparently related to the presence of a continuous lithospheric slab subducted near the northern coast of colombia the two largest earthquakes located at a significant distance from the bucaramanga nest present similar fault plane solutions moreover they also agree with those of the two largest earthquakes reported inside the nest this similarity suggests that the bucaramanga nest lies on the same subducted slab where the other earthquakes occur there is not enough shallow seismicity to define the location where the caribbean lithosphere subducted beneath the south american plate however the extension of the slab toward the surface inferred from the intermediate depth seismicity suggests that the subducted lithosphere may still be attached to the caribbean plate

seismicity and tectonics of the southeastern caribbean we present 33 new focal mechanisms for se caribbean earthquakes 1963 1988 we use these mechanisms in conjunction with 28 previously available mechanisms to distinguish between two models of plate boundary zone interaction in the se caribbean the trench trench transform and hinge faulting model and the right oblique collision model shallow 0 70 km and intermediate 70 200 km depth earthquakes occur in the study region we focus on the tectonic causes of these events and the motions they delineate the shallow earthquakes are in a broad linear zone which trends ne from the paria peninsula of venezuela towards barbados intermediate depth earthquakes cluster beneath and nw of the peninsula and deepen to the nw perpendicular to the ne trending shallow events the vertical distribution of the earthquakes suggests a slab with steep nw dip shallow dextral strike slip on e striking faults is restricted to a 60 km wide linear zone between the gulf of cariaco and the western margin of the gulf of paria dextral strike slip is active only as far east as the gulf of paria and not within or east of trinidad shallow thrust events with ene striking planes distributed between the araya peninsula and the gulf of paria indicate collision at crustal levels between south america and caribbean and that folding and thrusting are still active over a 60 km interval south of the araya paria isthmus active thrusting in venezuela corroborates predictions of transpression between caribbean and south america and discounts transtensional motions between the two plates in the se caribbean the conjunction of shallow thrust strike slip and normal earthquakes in the gulf of paria at around 62 3 may be the expression of unpartitioned oblique compressive deformation in the plate boundary zone intermediate 165 km h 70 km depth thrust and dip slip events within the nw dipping slab indicate that oceanic lithosphere probably originally attached to south america subducts to the nw beneath the caribbean plate shallow normal faulting events e and ne of trinidad are expressions of plate bending about near horizontal axes parallel to the lesser antilles subduction zone we conclude that the earthquake mechanisms provide strong support for the right oblique collision model of caribbean south american plate interaction

progressive back warping of a rider block atop an actively exhuming continental low angle normal fault abstract can continental low angle normal faults lanfs initiate and remain active at shallow although the mai iu fault is still active in the west to the east it dips 15 22 at the surface and has been abandoned in favor of the younger 42 7 dipping gwoira fault fault abandonment has caused part of the former hanging wall to be captured by the footwall this rider block s structure records progressive shallowing of the mai iu fault concurrent with slip we present 26al 10be cosmogenic nuclide burial dating along with a recently published u th sm he apatite and zircon age these provide the first direct dating of the gwoira conglomerate these data imply inception of the mai iu fault before 3 12 6 04 ma and inception of the gwoira fault between 2 29 and 5 62 ma bedding fault cut off angles suggest the mai iu fault did not initiate as a lanf rather it initially dipped 44 at the surface and was subsequently back rotated while active to its present 15 22 surface dip this suggests that continental lanfs initiate at moderate dips and evolve according to the rolling hinge process

active aseismic creep on the alto tiberina low angle normal fault italy the existence of active low angle normal faults has been questioned because the standard theory of fault mechanics precludes normal faults from slipping at low

large earthquakes and creeping faults faults are ubiquitous throughout the earth s crust the majority are silent for decades to centuries until they suddenly rupture and produce earthquakes with a focus on shallow continental active tectonic regions this paper reviews a subset of faults that have a different behavior these unusual faults slowly creep for long periods of time and produce many small earthquakes the presence of fault creep and the related microseismicity helps illuminate faults that might not otherwise be located in fine detail but there is also the question of how creeping faults contribute to seismic hazard it appears that well recorded creeping fault earthquakes of up to magnitude 6 6 that have occurred in shallow continental regions produce similar fault surface rupture areas and similar peak ground shaking as their locked fault counterparts of the same earthquake magnitude the behavior of much larger earthquakes on shallow creeping continental faults is less well known because there is a dearth of comprehensive observations computational simulations provide an opportunity to fill the gaps in our understanding particularly of the dynamic processes that occur during large earthquake rupture and arrest

present day tectonics of the southeastern caribbean and northeastern venezuela four telemetered seismic arrays were operated in northeastern venezuela during the summer of 1979 an analysis of the data collected has resulted in accurate locations for about 100 microearthquakes and four new focal mechanism solutions on the basis of these new data an in depth analysis of teleseismic data and geologic evidence we propose a new plate tectonic framework for the southeastern caribbean in this model underthrusting of the atlantic seafloor along the lesser antilles is extended to the southeast of trinidad the subducted slab is shown to dip northwesterly beneath trinidad and the caribbean sea penetrating depths of at least 150 km this subduction appears to terminate abruptly in the vicinity of the los bajos el soldado fault zone which trends wnw nw and is located in the gulf of paria west of trinidad geologic evidence shows right lateral strike slip rlss motion in this fault zone probably initiated in the late pliocene this wnw nw trending fault zone joins up with the e w trending el pilar casanay fault system in northeastern venezuela which also moves right laterally the el pilar fault is apparently offset by nnw trending faults that exhibit rlss motion as well as normal faulting this complex motion on an en echelon series of faults is interpreted to be the result of wnw motion of south america relative to the caribbean plate in our interpretation the e w component of relative motion is accommodated by rlss motion on e w faults and results in normal faulting on nnw sse faults the smaller component of n s convergence is reflected by rlss motion on nnw trending faults and is accommodated by some internal deformation the timing of the initiation of rlss motion on the los bajos fault zone and the known total amount of displacement on it as well as other geologic indicators lead us to suggest that this complex mode of strike slip motion began only a few m y ago and that the average rate of relative plate motion in this region is probably 0 9 0 2 cm yr

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

how continents break up insights from papua new guinea the woodlark basin in the western pacific forms a continuous system of active continental rifting evolving to well developed seafloor spreading thin sediment cover in the basin and a dominantly nonvolcanic rift phase permit basement fabric and structures to be imaged by swath mapping and seismic reflection data in the continental and oceanic parts of the basin magnetic isochrons indicate a single euler pole of opening for most of the basin history and allow us to infer the opening kinematics along the rifted margins in agreement with rigid plate tectonic models continental rifting initiated geologically synchronously at 6 ma along the length of the protomargins within a deforming plate boundary zone strain localization and seafloor spreading however developed in a time transgressive fashion from east to west within this zone of deformation spreading centers formed within the rheologically weaker protocontinental margins surrounded by stronger oceanic lithosphere in the solomon and coral seas the transition to spreading occurred after a rather uniform degree of continental extension 200 40 km both early and late stage rifting involved high and low angle normal faults we identify distinct styles in the transition from rifting to spreading which we refer to as nucleation propagation and stalling these breakup styles impart varyingly concordant to discordant relationships between the adjacent oceanic and continental rift structures continental transform margins which are or were juxtaposed against the ends of spreading centers show no evidence for thermal uplift or igneous underplating the initial spreading segments achieved much of their length at nucleation within rift basins separated along strike by accommodation zones with subsequent lengthening by spreading propagation into rifting continental crust this early propagation and the subsequent development of transform faults between initially nontransform spreading segment offsets produced rift and spreading segmentation boundaries that are not simply correlated the spreading centers nucleated approximately orthogonal in strike to the opening direction but as the protomargins were oblique to this direction nucleation jumps occurred in order to maintain the new spreading centers within the protomargins thus stepwise spreading nucleation in order to remain within a rheologically weak zone rather than rupturing of the lithosphere by stress concentration at the tip of a propagating ridge axis is the dominant form of the rifting to spreading transition in the woodlark basin