tertiary extensional faulting and evolving ductile brittle transition zones in the northern snake range and vicinity new insights from seismic data new geologic mapping and high resolution seismic data from east central nevada shed light on the evolution of the northern snake range decollement nsrd and its relation to the present basins and ranges we suggest that the nsrd originated in oligocene time as a flat 20 km wide 7 km deep zone of decoupling between localized ductile stretching below and more widely distributed high angle normal faulting above younger east dipping normal faults cut the western part of the nsrd forming the tilted fault blocks and highly asymmetric grabens that constitute the duck creek range duck creek valley and schell creek range spring valley and folded the nsrd to its present antiformal geometry these youngest faults flatten into pervasive subhorizontal reflectors at a depth of 10 to 15 km that we interpret as the youngest ductile brittle transition

compositional variation and the origin of deep crustal reflections abstract identifying the sources of crustal reflections is essential for deriving more geological information from deep crustal reflection profiles theoretical and model studies help place constraints on the role of compositional variation in producing deep crustal reflections analysis of laboratory derived velocities and densities of rock types typical for the continental crust indicates that reflection coefficients are generally small but significantly 17 of the possible reflection coefficients have magnitudes between 0 1 and 0 2 comparison between reflection coefficients derived from well logs and reflections observed in associated seismic profiles indicates that constructive interference associated with geological layering is at least as important as the magnitude of the reflection coefficients for producing detectable reflections constructive interference can increase reflection amplitude by two to three times but is limited to a relatively narrow range of layer thickness for a typical 10 40 hz seismic wavelet and typical crustal velocities of about 6 km s constructive interference occurs for layer thickness ranging between about 35 and 80 m layers thinner than 35 m interfere destructively if reflections result from compositional variation seismic models of hypothetical and observed geologic relations provide analogs for interpreting complex reflection patterns observed in deep crustal reflection profiles such models show reflection patterns similar to those observed in the reflection profiles the models indicate that the reflections could originate in the complexly deformed and intruded terranes that are common in the crystalline crust and it may not be necessary to appeal to unobserved phenomena such as special lamellae or fluid filled fractures to explain the reflections

intraplate extensional tectonics of the eastern basin range inferences on structural style from seismic reflection data regional tectonics and thermal mechanical models of brittle ductile deformation the general lack of a correlation between earthquakes and surface faulting in the interior of the western u s cordillera has motivated our efforts to evaluate the geometry structural style and mechanism of normal faulting characteristic of this region of intraplate extension to address the problem we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust rheological models of the continental crust were calculated to examine the possibility of shallow quasi plastic flow and its influence on faulting in the eastern basin range the western colorado plateau and the middle rocky mountains our data and interpretations have revealed the following styles of cenozoic deformation 1 steep to low angle dip normal faulting along the wasatch fault 2 low angle dip and listric normal faulting possibly associated with movement on preexisting thrusts 3 the occurrence of asymmetric mostly eastward tilted tertiary basins that are bounded by low to moderate dipping planar and listric faults and 4 at least three vertically stacked en echelon low angle reflections in the mid to upper crust that dip gently westward from 3 km beneath the wasatch plateau to over 15 km at the utah nevada border these reflections are interpreted as normal detachment faults the structural style of the pervasive low to moderate angle dipping faults cannot be easily reconciled with classic brittle failure theory but the interpreted termination of normal faults at or above the deeper low angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip an important observation based on interpretations of seismic reflection profiles is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits segment boundaries of the wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the precambrian uinta aulacogen suggesting that preexisting crustal structure partly controls the geometry of extension to examine the influence of ductile deformation quasi plastic flow was modeled for appropriate geotherms of the basin range and colorado plateau crusts these models provide constraints on the depth to the frictional quasi plastic transition that occurs as shallow as 8 km in the eastern basin range this depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes m 5 5 appear to nucleate at 10 15 km in or near the brittle ductile transition the rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region large scale low angle reflections may represent decollements and normal faults at the top of upper crustal zones of reduced shearing strength and higher ductility

calcrete in quaternary coastal dunes in southwestern australia a capillary rise phenomenon associated with plants abstract non pedogenic calcrete is forming as a thin sheet 10 50 cm thick just above the water table in the subsurface of a barrier dune terrain in south western australia the calcrete is forming in response to the regional evapo transpirative regime the annual rainfall is high but of short duration and it infiltrates rapidly into the sandy substrates relatively high air temperatures and evaporation coupled with wind stress combine to produce a water vapor deficit across the dune terrain the vegetation responds by drawing upon phreatic water and pellicular water of the vadose zone thereby precipitating caco3 where plants are not drawing phreatic water calcrete is developing by evaporation of water at the top of the zone of capillary rise various structure types of calcrete are developed in the study area namely rhizoconcretionary mottled massive and laminar forms these occur in a definite stratigraphic sequence related to maturity of profile vegetation cover and thickness of the vadose zone rhizoconcretionary calcrete forms in the vadose zone mottled and massive calcrete frequently capped by laminar calcrete develop in the zone of capillary rise these latter calcrete types form a sheet that is ubiquitous and parallel to the water table since it is a feature of the water table the calcrete sheet may potentially transect regional stratigraphic boundaries and unconformities at the local level the various structural types of calcrete may he used as paleowater table indicators

the snake range decollement an exhumed mid tertiary ductile brittle transition the snake range decollement srd in east central nevada separates supracrustal rocks extended by normal faulting from ductilely deformed igneous and metamorphic rocks a well known stratigraphy unaffected by earlier faulting permits analysis of both upper and lower plate strain leading to a better understanding of how vastly different rock types and deformational styles are juxtaposed along low angle faults in metamorphic core complexes middle cambrian to permian upper plate rocks are cut by two generations of ne trending east directed normal faults both generations were initiated as high angle 60 planar faults that flattened abruptly into the srd and rotated domino style to low angles yielding a total rotation of bedding of about 80 90 faulting is tertiary in age as 35 m y old volcanic rocks are involved and resulted in about 450 500 extension in a n55w s55e direction the srd developed as a subhorizontal surface 6 7 km deep at the top of the cambrian pioche shale lower plate granitic rocks and their late precambrian cambrian metamorphic country rocks were involved in progressive ductile to brittle extension at low greenschist grade forming a penetrative subhorizontal foliation and n55 70w lineation that increases in intensity eastward and upward toward the srd stretching and thinning in the lower plate is coaxial and comparable in magnitude to upper plate extension and is interpreted as synchronous k ar ages ranging from 20 to 40 m y in the lower plate suggests the n snake range represents a tertiary thermal anomaly we conclude that the srd developed as a ductile brittle transition zone at 6 7 km depth gravity data suggests that the gently domed srd is cut by younger basin and range faults but the geology of adjacent ranges suggests that the srd does not continue more than 60 km in any given direction the lack of stratigraphic omission across the srd rules out large amounts of movement on a surface that originally cut downsection and we suggest that extensional detachment faults such as the srd can be developed locally as boundaries between brittlely extended rocks and underlying ductile extension and intrusion

implications of selected subsurface data on the structural form and evolution of some basins in the northern basin and range province nevada and utah

the andean subduction zone between 22 and 25 s northern chile precise geometry and state of stress abstract one year of seismicity recorded by a local network is used to obtain more precision about the geometry and the stress regime of the andean subduction between 22 and 25 s in the northern chile seismic gap a sharp image of the wadati benioff zone wbz is obtained down to 270 km in depth a seismically quasi quiescent zone is observed in the wbz below the volcanic arc between 150 and 210 km in depth hypocentres of distant intermediate depth earthquakes located with the local network are compared with worldwide seismic network hypocentres in order to evaluate the accuracy of the wbz image at depth greater than 100 150 km in depth no shallow microearthquakes have been observed in the continental crust but some seismic activity is likely to occur locally at the deep root of the atacama fault the stress field and the characteristics of faulting along the subducted slab are investigated underthrusting and localized reverse faulting earthquakes define the seismically coupled plate interface from 20 to 50 km in depth locked zone downdip intra slab normal faulting prevails tensile zone but some strike slip faulting is observed a transition between normal faulting with variable fault azimuth and normal faulting with nearly homogeneous nnw to nw oriented fault plane is found at about 80 km in depth it is found that the stress axes 1 and 3 in the locked zone are oriented in the convergence direction 75 80 e downdip in the tensile zone 3 has a mean azimuth 60 65 e there the slab is hence submitted to a tensional force slab pull oblique relatively to the convergence the transition between seismic underthrusting and intraplate normal faulting downdip occurs at the depth where the continental moho encounters the wadati benioff zone suggesting that a relationship exists between seismic coupling and the presence of continental crust at the plate interface the pre seismic state of this segment of the andean subduction zone is confirmed by the occurrence of strong earthquakes located by the global network around the presumed rupture area and by the stress regime found along the wadati benioff zone

fine details of the wadati benioff zone under indonesia and its geodynamic implications using about 5200 handpicked p s pp sp pcp and scp phases from digitally recorded seismograms together with international seismological centre reported phases we obtain improved hypocentral locations for 1790 earthquakes 983 of them having 90 confidence limits 50 km in the period 1962 to september 1996 along the indonesian subduction zone we use an improved joint hypocenter determination method in which the solution is more stable than in the original one we show that for deep earthquakes use of the core reflected phases is as good as use of depth phases and since such core phases often have larger amplitudes and sharper onsets than the depth phases their arrival times can be read very accurately the positions of the relocated hypocenters show that 1 a portion of the indonesian arc between 110 e and 123 e longitude and deeper than 500 km is dipping southward at an angle of 75 that is in a direction opposite to the upper part of the north dipping slab and 2 east of about 108 e the seismic zone is wider near 670 km than near the 500 km depth the first suggests southward lateral flow in the mantle relative to the plate motion vector here from the contortion of the seismic zone along the eastern portion of the indonesian arc we can estimate the average lateral shear strain rate in the 300 to 670 km depth range to be of the order of 10 16s 1 over the last 10 20 myr

the snake range decollement interpreted as a major extensional shear zone geological and geophysical constraints suggest that the snake range decollement of east central nevada is a major tertiary low angle normal fault zone this interpretation is consistent with all existing data and alleviates problems that result if large displacement across the decollement is excluded miller et al 1983 based on published surface geology and cocorp seismic reflection profiling we have constructed cross sections that suggest approximately 60 km of normal displacement on the decollement advantages of this interpretation over models that exclude large displacement are that it provides for overburden consistent with kyanite grade metamorphism of footwall rocks predicts reasonable crustal thicknesses before and after extension without invoking unexposed mantle derived tertiary intrusions at depth and explains contrasting metamorphism and structural styles of hanging wall and footwall without requiring an extreme geothermal gradient during regional metamorphism and extensional strain the present domal shape of the decollement is interpreted to result at least partly from reverse drag above an underlying younger detachment related to much of the extension in ranges to the west of the snake range

contributions to the history of geomorphology and quaternary geology an introduction this special publication deals with various aspects of the histories of geomorphology and quaternary geology in different parts of the world geomorphology is the study of landforms and the processes that shape them past and present quaternary geology studies the sediments and associated materials that have come to mantle much of earth s surface during the relatively recent pleistocene and holocene epochs geomorphology with its concern for earth s surface features and processes deals with information that is much more amenable to observation and measurement than is the case for most geological work quaternary geology focuses mostly but not exclusively on the earth s surficial sedimentary cover which is usually more accessible than the harder rocks of the deeper past institutionally geomorphology is usually situated alongside or within academic departments of geology or geography in most english speaking countries its links are more likely to be with geography but in the united states these connections are usually shared between geography and geology although rarely in the same institution in leading institutions everywhere strong links exist between geomorphology and such cognate disciplines as soil science hydrology oceanography and civil engineering although nominally part of geology quaternary geology also has strong links with geography and with those disciplines such as climatology botany zoology and archaeology concerned with environmental change through the relatively recent past given that geomorphology concerns the study of the earth s surface i e landforms and their origin evolution and the processes that shape them and that the uppermost strata are in many cases of pleistocene and holocene age it is unsurprising that this special publication should deal promiscuously with topics in both geomorphology and quaternary studies this particular selection has been developed from a nucleus of papers presented at a conference on the histories of geomorphology and quaternary geology held in the baltic states

seismic reflection evidence for seismogenic low angle faulting in southeastern arizona focal mechanisms for large m 6 earthquakes in extensional terranes suggest that seismogenic normal faults have dips that range from approximately 30 degree to approximately 70 degree geologic relations suggest that low angle faults have accommodated large scale upper crustal extension these disparate observations are often reconciled by arguments that low angle faults move aseismically or rotate to low angles from initially high angles seismic reflection data from the tucson basin in southeast arizona image a low angle normal fault the santa rita fault that crops out along the trend of late quaternary fault scarps caused by large magnitude m approximately 6 7 7 6 earthquakes velocity independent dip analysis from shot records of the santa rita fault indicates that it has a true dip of approximately 20 degree to a depth of at least 6km this observation suggests that low angle extensional faults indeed may be seismogenic and that actual mechanisms for accommodation of upper crustal extension depend on local conditions of stress and preexisting geologic structure

active normal faulting and crustal extension a world wide review of fault plane solutions and focal depths for large normal faulting earthquakes on the continents shows that the overwhelming majority of such earthquakes nucleate in the depth range 6 15 km on faults dipping between 30 and 60 in the few cases where levelling or seismic data are good enough these normal faults are shown to be approximately planar from the surface to their nucleation depth at the base of the brittle crust there is evidence that in some cases as a result of the enormous transitory increase in strain rate during large earthquakes rupture continues into the normally ductile lower crust on surfaces with substantially gentler dips these low angle surfaces may be analogous to some of the detachments seen in metamorphic core complexes of the western usa but the nature of the motion on them depends on strain rate as well as on rheological contrasts between the detachments and the blocks on either side such contrasts however are unlikely to introduce substantial curvature to an originally planar shear zone if the observed spread in active normal fault dips is caused by rotation of the faults and the blocks they bound during extension then a maximum value of 1 7 can be accommodated by seismic activity on a single generation of normal faults with continued extension either a new generation of steeper faults cutting the rotated first faults is likely to form or the deformation will continue aseismically on faults dipping at less than 30 large earthquakes have not been observed to nucleate on very low angle 20 normal faults within the continental crust anywhere in the world such faults can of course move aseismically but are unlikely to do so on a large scale within the upper crust in areas where steep normal faults are seismically active thus extensional models that require concentrated simple shear on large sub horizontal faults within the brittle upper crust will also require a spatial separation between aseismic very low angle faulting and seismic highangle faulting since seismogenic high angle faults dominate the topography of extending regions upper crustal very low angle faulting if it occurs on a large scale presumably does so in areas that are relatively fiat

internal tectonic structure of the central american wadati benioff zone based on analysis of aftershock sequences 1 relocated engdahl et al 1998 global seismological data for 10 aftershock sequences were used to analyze the internal tectonic structure of the central american subduction zone the main shocks of several of these were the most destructive and often referenced earthquakes in the region e g the 1970 chiapas 1983 osa 1992 nicaragua 1999 quepos 2001 el salvador earthquakes the spatial analysis of aftershock foci distribution was performed in a rotated cartesian coordinate system x y z related to the wadati benioff zone and not in a standard coordinate system h are latitude longitude focal depth respectively available fault plane solutions were also transformed into the plane approximating the wadati benioff zone the spatial distribution of earthquakes in each aftershock sequence was modeled as either a plane fit using a least squares approximation or a volume fit with a minimum thickness rectangular box the analysis points to a quasi planar distribution of earthquake foci in all aftershock sequences manifesting the appurtenance of aftershocks to fracture zones geometrical parameters of fracture zones strike dip and dimensions hosting individual sequences were calculated and compared with the seafloor morphology of the cocos plate the smooth character of the seafloor correlates with the aftershock fracture zones oriented parallel to the trench and commonly subparallel to the subducting slab whereas subduction of the cocos ridge and seamounts around the quepos plateau coincides with steeply dipping fracture zones transformed focal mechanisms are almost exclusively 90 of normal character

tertiary structural development of selected valleys based on seismic data basin and range province northeastern nevada reflexion seismic data in railroad diamond mary s river and goshute valleys provide information on their structural development that cannot be deduced solely from outcrop and well data these valleys contain tertiary sediments that in dip section define an asymmetrical basin bounded along the eastern flank by a major listric normal fault with about 3 0 4 5 km of displacement the west flank is defined by a gentle east dipping ramp seismically the trace of the listric fault is interpreted to dip westward and sole into the palaeozoic section exploiting regionally recognized mesozoic decollement surfaces the tertiary depocentre adjacent to this fault shifted from west to east with continued slippage through time the greatest movement occurring in the miocene and post miocene in the strike direction the valleys are separated into at least two subbasins by an east west structurally high axis the axis is postulated to be the result of a tear fault associated with movement along the listric normal fault tertiary stratigraphy varies between valleys and between sub basins in a given valley all the valleys contain miocene and younger rocks however not all sub basins contain the pre miocene section suggesting a complex scheme of structural development

long term landscape evolution in australia the main features of the australian physical landscape are of the order of 107 108 years old this contradicts the widely held view that little of the earth s topography predates the quaternary and that erosion cycles are carried to planation within tens of millions of years much of the australian landscape must have developed over similar timescales to that of the tectonic evolution of the continent itself the study of the geomorphology of such ancient terrains may therefore be seriously deficient unless it is considered within the context of continental scale tectonic development application of this approach shows that there are strong links between the geomorphology of australia and plate movements ocean spreading plate convergence tectonostratigraphic terranes orogenesis and epeirogenesis the most important factor contributing to the survival of ancient landscapes in australia is the low rate of denudation which the continent has experienced during the mesozoic and cenozoic this is largely a consequence of orogenic stability although the absence of significant quaternary glaciation may also be of importance however in order for landforms to have survived over such timespans denudation must not only have been low but must also have been highly localized over space and time this has been the case both on a regional scale with long term denudation rates of 0 2 m ma 1 in central australia contrasting with higher rates along the continental margins and on a local scale with denudation confined to valleys leaving divides and interfluves almost unscathed

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

migration of the seismic aseismic transition during uniform and nonuniform extension of the continental lithosphere we investigate the thermal and seismic implications of two extreme configurations of lithospheric extension one extreme treats the extension as being approximately uniform with depth the upper crust extending by the rotation of fault bounded blocks above a diffusely deforming lower crust in the other extreme crustal extension occurs only above a low angle fault dip less than about 10 very low angle shear zones that initially move aseismically within a uniformly extending lithosphere may be raised through the depth range that is normally seismically active without cooling appreciably the condition for this to occur is that extension should take place faster than about 10 14 s in contrast extension accommodated entirely above a very low angle shear zone rapidly brings that zone to temperatures at which it should be seismically active even if initial movement on it were aseismic the different thermal histories implied by the two configurations of extension offer the possibility of distinguishing between modes of extension by observations of temperature time and pressure temperature paths on rocks close to low angle faults

recent plumbing system of the krakatau volcano revealed by teleseismic earthquake distribution spatial and temporal analysis of global seismological data 1964 2005 reveals a distinct teleseismic earthquake activity producing a columnar like formation in the continental wedge between the krakatau volcano at the surface and the subducting slab of the indo australian plate these earthquakes occur continuously in time are in the body wave mb magnitude range 4 5 5 3 and in the depth range 1 100 km the krakatau earthquake cluster is vertical and elongated in the azimuth n30 e suggesting existence of a deep rooted fault zone cutting the sunda strait in the ssw nne direction possible continuation of the fault zone in the sw direction was activated by an intensive 2002 2003 aftershock sequence elongated in the azimuth of n55 e beneath the krakatau earthquake cluster an aseismic gap exists in the wadati benioff zone of the subducting plate at the depths 100 120 km we interpret this aseismic gap as a consequence of partial melting inhibiting stress concentration necessary to generate stronger earthquakes whereas the numerous earthquakes observed in the overlying lithospheric wedge beneath the volcano probably reflect magma ascent in the recent plumbing system of the krakatau volcano focal depth of the deepest events 100 km of the krakatau cluster constrains the location of the primary magma generation to greater depths the ascending magmatic fluids stress fault segments within the sunda strait fault zone and change their friction parameters inducing the observed tectonic earthquakes beneath krakatau

the geometrical evolution of normal fault systems abstract the purpose of this paper is to examine the kinematic behaviour of normal fault systems and see what general conditions govern their geometrical evolution we pay particular attention to seismological and surface data from regions of present day active normal faulting as the instantaneous three dimensional geometry at the time of fault movement is better known in active regions than in areas where the faults are now static most normal faults are concave upward or listric this shape can be produced by geometric constraints either because the faults reactivate curved thrusts or because they must be curved to accommodate rotations another effect which will produce curved faults is the variation of rheology with depth brittle failure at shallow depths produces less fault rotation than does distributed creep in the lower part of the crust an important geometric feature of normal faulting is the uplift of the footwall the amount of such uplift is related not only to the elastic properties of the lithosphere but also to the throw and dip of the fault a striking feature of active normal faults is that they occur in groups in which all the faults dip in the same direction this behaviour arises because the faults cannot intersect if they do one must cease to be active the rotation which such fault systems produce reduces the dip of the faults until a new steeply dipping fault is formed once a new fault cuts pre existing faults the earlier faults become locked and a new set of faults must propagate rapidly across the whole region involved many of these geometric constraints also apply to thrust faulting

seismic response to recent tectonic processes in the banda arc region abstract internal structure and recent tectonics of the geologically complex banda arc region latitude 1 9 s longitude 124 135 e have been studied by means of the analysis of global seismological data relocated hypocentral determinations of the international seismological center ehb data and fault plane solutions of the global centroid moment tensor project have been used together with previously published information on regional geology and dynamics the analysis considered earthquakes in the lithospheric wedges above subducting slabs and earthquakes in the uppermost portions of the wadati benioff zones focal depth

identification of multiple generations of crosscutting domino style faults insights from seismic modeling seismic modeling and geometric analysis provide clues for identification of multiple generations of cross cutting planar rotational faults in seismic reflection profiles many elements characteristic of this structural geometry are apparent although contorted at shallow levels in unmigrated and migrated synthetic seismograms structural features potentially signaling an earlier generation of rotational faulting within a tilt block terrain include first generation fault segments intrablock terminations of horizons steep dips of prefault strata anomalously high fault strata intersection angles composite basin geometry truncations of early synextension sedimentary wedges and anomalous crustal thinning evaluation of published profiles from the bay of biscay and galicia bank indicates that the crosscutting domino fault model probably is not applicable in these areas as previously suggested while the model may be appropriate for the basement structure of spring valley nevada seismic evidence is equivocal

seismic modeling study of the earth s deep crust we use seismic modeling methods to validate the interpretation of deep crust seismic exploration an approximation of the stacked section is obtained with the nonreflecting wave equation and the exploding reflector approach using this technique and ray tracing algorithms we obtain a geological model by comparing the synthetic section with the real stacked section an isotropic constitutive equation is assumed in this phase the exact synthetic stacked section is then obtained by applying the standard processing sequence to a set of synthetic common shot profiles computed with the variable density acoustic wave equation we introduce elliptical p wave anisotropy and the effects of small scale inhomogeneities by using a von karman autocovariance probability function that simulates scattering q effects verification of the geological model by poststack migration constitutes an additional test the methodology which is suitable for areas of complex geology is applied to a seismic line acquired in the nort

mid pleistocene basalt from the seguam volcanic center central aleutian arc alaska local lithospheric structures and source variability in the aleutian arc recent geophysical and tectonic studies make it possible to examine the relation between local structural and tectonic evolution of the modern aleutian arc and the sources and processes that produce the broad spectrum of aleutian basaltic lavas mid pleistocene basalt from the seguam island volcanic center in the central aleutian arc erupted through strongly extended arc crust contains 4 8 mgo is low in k2o 18 and is distinguished from aleutian basalt erupted through less extended crust by a low abundance of incompatible and rare earth elements ree and low sr and pb content the ba la 47 72 and pb la 0 7 1 5 ratios are high incompatible element ratios are variable and ree patterns cross one another the 87sr 86sr ratio 0 70365 0 00003 is the highest value obtained from the aleutian islands also 206pb 204pb 18 91 0 02 207pb 204pb 15 60 0 02 and 208pb 204pb 38 51 0 04 ratios are among the most radiogenic values known whereas the 143nd 144nd ratio 0 51297 0 00003 is among the lowest measured low abundances of incompatible elements high ba la and pb la ratios uniform sr nd and pb isotope ratios and high eruptive temperatures 1160 c suggest that basalt erupted at seguam ascends through extended subarc lithosphere without extensive chemical exchange with the upper mantle or lower crust partial melting of geochemically distinct portions of an isotopically uniform source region best explains observed geochemical characteristics trace element abundances rule out an eclogite source and suggest instead a peridotite dominated source with 5 10 of a subducted sediment component this peridotite may reside in the mantle wedge or in the amlia fracture zone afz of the pacific plate that was subducted beneath seguam 1 ma in addition the afz possibly enhanced the subduction of terrigenous sediment into the source region beneath seguam and this extra terrigenous component may explain the ba la pb la and sr nd and pb isotope ratios the distinctive composition of seguam basalt reflects a complex set of physical conditions including fracture zone subduction and intraarc extension that affected both the source region and thermo chemical history of magma ascent local structural and tectonic factors impact profoundly the nature of island arc magma sources and processes models that propose a common parental arc magma type or common processes on a global or arcwide scale are too generalized to explain the diversity actually observed in the aleutian arc

seismically active column and volcanic plumbing system beneath the island arc of the izu bonin subduction zone summary a detailed spatio temporal analysis of teleseismic earthquake occurrence mb 4 0 along the convergent margin of the izu bonin mariana arc system reveals an anomalously high concentration of events between 27 and 30 5 n beneath a chain of seamounts between tori shima and nishino shima volcanoes this seismicity is dominated by the 1985 1986 earthquake swarm represented in the engdahl van der hilst buland database by 146 earthquakes in the body wave magnitude range 4 3 5 8 and focal depth range 1 100 km the epicentral cluster of the swarm is elongated parallel to the volcanic chain available focal mechanisms are consistent with an extensional tectonic regime and reveal nodal planes with azimuths close to that of the epicentral cluster earthquakes of the 1985 1986 swarm occurred in seven time phases seismic activity migrated in space from one phase to the other earthquake foci belonging to individual phases of the swarm aligned in vertically disposed seismically active columns the epicentral zones of the columns are located in the immediate vicinity of seamounts suiyo and mokuyo recently reported by the japanese meteorological agency as volcanically active the three observations episodic character of earthquake occurrence column like vertically arranged seismicity pattern and existence of volcanic seamounts at the seafloor above the earthquake foci led us to interpret the 1985 1986 swarm as a consequence of subduction related magmatic and or fluid activity a modification of the shallow earthquake swarm magmatic model of d hill fits earthquake foci distribution tectonic stress orientation and fault plane solutions the 1985 1986 deep rooted earthquake swarm in the izu bonin region represents an uncommon phenomenon of plate tectonics the portion of the lithospheric wedge that was affected by the swarm should be composed of fractured rigid brittle material so that the source of magma and or fluids which might induce the swarm should be situated at a depth of at least 100 km in the aseismic part of the subduction zone

relations between normal fault geometry tilting and vertical motions in extensional terrains an example from the southern gulf of suez abstract earthquakes and data from subsurface oil exploration suggest that large active normal faults in the southern gulf of suez are approximately planar with dips of 30 40 from the surface to around 10 km depth these faults and the blocks they bound appear to rotate about a horizontal axis as they move causing tilting this tilting is seen both in young vertical movements of the coastline such as raised beaches and marshlands and in the distribution of middle miocene marine rocks which are uplifted to elevations of 400 500 m in footwalls of faults and found at depths of around 3500 m in the adjacent grabens the absolute amplitude of the observed vertical motions can be approximately modelled by planar rotating normal faults that impose a saw tooth topography on a regional subsidence caused by crustal and lithospheric thinning the observations required for this simple model are the present day fault dip the amount of tilting and the width of the rotating blocks the virtues of the model are its simplicity and its compatibility with our knowledge of how large active normal faults move elsewhere on the continents

earthquake clustering in the tectonic pattern and volcanism of the andaman sea region abstract seismicity pattern in the andaman sea region has been analysed to interpret recent dynamics of regional submarine volcanic provinces hypocentral determinations and focal mechanisms of available global seismological data have been used frequent occurrence of earthquake swarms proved to be an important characteristic of seismicity pattern in the investigated region epicenters of the earthquake swarms are situated almost exclusively along a narrow belt forming the easternmost limit of the epicentral area of the andaman sea region the southern zone of the belt between 6 and 10 5 n is trench parallel and coincides with the northward prolongation of the sumatra volcanic arc the northern zone of the belt between 10 5 and 13 n is deflected by about 45 northeast and precisely follows the complicated zigzag pattern of the rift system in the middle of the andaman basin earthquake occurrence in these two zones the northern and the southern differs by several aspects by shape of the epicentral zones of individual swarms by focal mechanisms and by response to the 2004 sumatra andaman great earthquake these differences together with available information on composition of magmas found at the seafloor lead us to the conclusion that the swarms of the southern zone are induced by intrusions of subduction generated calc alkaline magmas whereas the swarms in the northern zone by intrusions of basaltic magmas associated with the seafloor spreading earthquake swarm occurrence defines a brittle seismogenic layer at depths between 9 and 35 km excludes the existence of large magma reservoirs in respective depth interval and puts their hypothetical location to a greater depth episodes of magma ascent from deeper reservoir to shallow magma chamber depth

tension fractures and extensional tectonics a characteristic pattern of normal faults that we have studied in several regions illustrates successive geometries relating to increasing amounts of extension from 10 50 gulf of suez egypt to 50 100 western gulf of california mexico up to 200 southern basin and range usa tension fractures formed at an early stage of extension have an essential role the resulting structure is described as a multiple tilted packs of cards model in which rotational deformation within the major blocks is accommodated by normal faulting on previously formed tension fractures thus defining smaller blocks of second and third generation

strontium isotopic and selected trace element variations between two aleutian volcanic centers adak and atka implications for the development of arc volcanic plumbing systems major and trace element concentrations and initial 87sr 86sr ratios of lavas from the aleutian volcanic centers of adak and atka have been used to study the evolution of their respective lithospheric plumbing systems the centers are within 150 km of one another and show similar overall silica ranges 47 67 but adak 40 km3 is smaller than atka 200 km3 adak s lavas are chemically and isotopically heterogeneous 87sr 86sr 0 70285 0 70330 and two units contain lithospheric xenoliths the lavas of the much larger atka on the other hand have much less variability in major and trace elements as well as 87sr 86sr 0 70320 0 70345 we suggest that these characteristics are a measure of the relative maturity and cleanliness of the lithospheric plumbing systems that supply magma to these centers because aleutian volcanic centers often remain fixed for relatively long periods of time 5 m y once established magmatic passageways are repeatedly used young plumbing systems are relatively cool and contain large amounts of wallrock contaminant and ascending magmas undergo contamination as well as concurrent crystallization and fractionation with time however heat and mass transfer between ascending magmas and wallrock produce thermal and chemical boundary layers that insulate subsequent magmas in effect the plumbing system matures the chemical heterogeneity displayed by young dirty systems like adak reflects not only the magma source but also the wallrock encountered during ascent and possibly the effects of extensive crystal fractionation thus it is the petrologic data of mature clean systems like atka that yield the most direct and unambiguous information on the ultimate origin of the lavas and their near surface evolution

seismic history and seismotectonics of the sunda arc historic records of the last 300 years reveal two great interplate earthquakes 1833 mw 8 1861 mw 8 8 which ruptured major segments of the sumatra fore arc in western indonesia a significant percentage of interplate slip along this portion of the plate boundary appears to occur seismically the ends of these rupture zones are coincident with clusters of large ms 7 and moderate 6 ms 7 shocks and with heterogeneities in the plate interface as inferred from geologic and geophysical data the northern extent of the 1833 rupture zone is coincident with a group of moderate and large earthquakes a structural arch in the fore arc and a fracture zone on the subducted plate the southern portion is coincident with a very prominent cluster and the along strike extension of a sediment filled trough on the subducted plate prior to this study sumatra was characterized as relatively aseismic as inferred from the lack of great earthquakes in the instrumental record of this century java and the lesser sunda islands had major ms 6 earthquakes in the historic record but none are of the same magnitude as the great events near sumatra there is no clear evidence that shocks in the java fore arc were interplate events yet geophysical data indicate recent uplift of the trench slope break and a change in strike of the trench axis where the northernmost flank of the roo rise a prominent bathymetrie high appears to be interacting with the fore arc this feature was formed at a ridge crest and is more buoyant and thus more difficult to subduct than the surrounding seafloor there is no increase in seismicity associated with this tectonism however that may indicate a lack of contact between the crystalline portions of the plates at this time we infer that a majority of slip on the plate interface near java occurs aseismically the variation in occurrence of great interplate earthquakes along the sunda arc may be interpreted in terms of a model that attributes variation in interplate coupling to the age of the subducted lithosphere great interplate earthquakes occur near sumatra where the age of the youngest crust is 46 ma the characteristics of the sunda arc and analogies with pacific arcs imply that the entire length of sumatra has the potential to produce great thrust earthquakes its seismic potential should be considered high as there has been no recurrence of the great events of 1833 and 1861 in contrast no such events have been reported off java and the lesser sunda islands where the oldest crust is 152 ma the plate interface near java and the lesser sunda islands should be considered to have low seismic potential

petrogenesis of oceanic andesites the andesites and cogenetic rocks of the intraoceanic island arc systems vary in silica content from 49 50 sio2 to about 76 sio2 the most primitive compositions having contents of about 50 sio2 are the basaltic andesites which are primary compositions generated by partial melting of the uppermost section of the subducted oceanic crust the oceanic andesites cannot have assimilated material from the continental crusts but their petrogenesis bears on the origin of the continental crust the fractionation of andesitic magmas occurs at relatively low pressures within the stability range of plagioclase basaltic andesites fractionate mainly orthopyroxene plagioclase while andesites fractionate clinopyroxene orthopyroxene plagioclase the petrogenesis of the continental andesites is more complex as the primary andesites contain sedimentary material from the continental crusts

seismicity pattern an indicator of source region of volcanism at convergent plate margins abstract the results of detailed investigation into the geometry of distribution of earthquakes around and below the volcanoes korovin cleveland makushin yake dake oshima lewotobi fuego sangay nisyros and montagne pelee at convergent plate margins are presented the isc hypocentral determinations for the period 1964 1999 based on data of global seismic network and relocated by engdahl van der hilst and buland have been used the aim of this study has been to contribute to the solution of the problem of location of source regions of primary magma for calc alkaline volcanoes spatially and genetically related to the process of subduction several specific features of seismicity pattern were revealed in this context i a clear occurrence of the intermediate depth aseismic gap idag in the wadati benioff zone wbz below all investigated active volcanoes we interpret this part of the subducted slab which does not contain any teleseismically recorded earthquake with magnitude greater than 4 0 as a partially melted domain of oceanic lithosphere and as a possible source of primary magma for calc alkaline volcanoes ii a set of earthquakes in the shape of a seismically active column sac seems to exists in the continental wedge below volcanoes korovin makushin and sangay the seismically active columns probably reach from the earth surface down to the aseismic gap in the wadati benioff zone this points to the possibility that the upper mantle overlying the subducted slab does not contain large melted domains displays an intense fracturing and is not likely to represent the site of magma generation iii in the continental wedge below the volcanoes cleveland fuego nisyros yake dake oshima and lewotobi shallow seismicity occurs down to the depth of 50 km the domain without any earthquakes between the shallow seismically active column and the aseismic gap in the wadati benioff zone in the depth range of 50 100 km does not exclude the melting of the mantle also above the slab iv any earthquake does not exist in the lithospheric wedge below the volcano montagne pelee the source of primary magma could be located in the subducted slab as well as in the overlying mantle wedge v frequent aftershock sequences accompanying stronger earthquakes in the seismically active columns indicate high fracturing of the wedge below active volcanoes vi the elongated shape of clusters of epicentres of earthquakes of seismically active columns as well as stable parameters of the available fault plane solutions seem to reflect the existence of dominant deeply rooted fracture zones below volcanoes these facts also favour the location of primary magma in the subducting slab rather than in the overlying wedge we suppose that melts advancing from the slab toward the earth surface may trigger the observed earthquakes in the continental wedge that is critically pre stressed by the process of subduction however for definitive conclusions it will be necessary to explain the occurrence of earthquake clusters below some volcanoes and the lack of seismicity below others taking into account the uncertainty of focal depth determination from global seismological data in some regions

structure of wadati benioff zones and volcanism produced by the process of subduction abstract the spatial correlation of the distribution of earthquake foci and of the position of active volcanoes in the regions of convergent plate margins allowed us to distinguish three main types of active volcanism spatially and genetically related to the process of present subduction volcanism of calc alkaline composition situated in a linear chain of volcanoes above the intermediate depth aseismic gap of the wadati benioff zone volcanism of variable composition bound to deep fracture zones induced by subduction in the overlying lithospheric plate and volcanism of tholeiitic composition related to transform faults forming closures of subduction zones depth ranges of the possible source of primary magma for individual volcanoes associated with the intermediate aseismic gap are given for the regions of central america andean south america kamchatka kurile islands hokkaido vanuatu archipelago and tonga kermadec islands

earthquake occurrence along the java trench in front of the onset of the wadati benioff zone beginning of a new subduction cycle 1 spatial distribution of earthquake foci in the central part of the sunda arc was analyzed using global seismological data the analysis revealed the existence of a distinct strip of earthquakes distributed along the java trench separated by a trench parallel 50 150 km wide aseismic link from seismicity belonging to the wadati benioff zone of the recently subducting slab the seismicity pattern at the plate margin corresponds well with the morphology of the seafloor the along trench seismicity correlates with the position of the java trench and the onset of the wadati benioff zone correlates with the java trough the gps measurements performed in three analogous structural units of the neighboring sumatran convergent margin point to substantial differences in velocities of movement of the respective units and correlate with the spatial distribution of earthquake foci we interpret the distinct along trench seismicity as a consequence of the onset of a new subduction cycle due to the rapid convergent movement of the indo australian plate relative to the aseismic lithospheric link

the evolution of the gulf of corinth greece an aftershock study of the 1981 earthquakes summary a preliminary study of the aftershocks of three earthquakes that occurred near to corinth greece in 1981 is combined with observations of the morphology and faulting to understand the evolution of the eastern gulf of corinth the well located aftershocks form a zone 60km long and 20km wide they do not lie on the main fault planes and are mostly located between the north dipping faulting on which the first two earthquakes occurred and the south dipping faulting associated with the third event a cluster of aftershocks also lies in the footwall of the eastern end of the south dipping fault of the third event morphologically it is observed that in the evolution of the eastern gulf of corinth antithetic faulting apparently predates the appearance of the main faulting at the surface this evolution can be explained by motion on a deep seated shallow angle aseismic normal fault a model based on such a fault also accounts for the aftershock distribution of the 1981 earthquakes