terrestrial manganese 53 a new monitor of earth surface processes we report the first systematic study of the terrestrial cosmogenic radionuclide manganese 53 t1 2 3 7 ma measured in thirteen samples from nine dolerite surfaces in the dry valleys antarctica the terrestrial manganese 53 concentrations correlate well with cosmic ray produced helium 3 and neon 21 concentrations in the same samples implying that the manganese 53 is produced continuously in situ and retained quantitatively over millions of years the terrestrial manganese 53 production rate determination normalized to iron the only important target element and to high latitude and sealevel yields a value of p53 103 11 atoms yr 1 g fe 1 this is consistent with the theoretical value of 120 18 atoms yr 1 g fe 1 obtained from modeling calculations our results show that the manganese 53 concentrations in bulk terrestrial rocks can be used to monitor earth surface processes on time scales exceeding 10 my 2006 elsevier b v all rights reserved

cosmogenic nuclides in the kirin chondrite abstract radioactive and stable cosmogenic nuclides have been determined in the various fragments of the kirin chondrite h5 experimental results obtained from our samples are described and compared with similar data obtained in other laboratories cosmogenic 54 mn 22 na 60 co 26 al 53 mn 40 k in the metal phase and light noble gases were measured based on these data the irradiation history of this meteorite can be explained in terms of 1 a multi stage exposure which involves a first stage irradiation for 10 7 years and a second stage for about 4 10 5 years and 2 depth effects in the productions of the nuclides in 4 second stage and 2 first stage geometries these conclusions are consistent with those of our previous work which was based on a limited number of samples

vertical tectonic deformation associated with the san andreas fault zone offshore of san francisco california abstract a new fault map of the shelf offshore of san francisco california shows that faulting occurs as a distributed shear zone that involves many fault strands with the principal displacement taken up by the san andreas fault and the eastern strand of the san gregorio fault zone structures associated with the offshore faulting show compressive deformation near where the san andreas fault goes offshore but deformation becomes extensional several km to the north off of the golden gate our new fault map serves as the basis for a 3 d finite element model that shows that the block between the san andreas and san gregorio fault zone is subsiding at a long term rate of about 0 2 0 3 mm yr with the maximum subsidence occurring northwest of the golden gate in the area of a mapped transtensional basin although the long term rates of vertical displacement primarily show subsidence the model of coseismic deformation associated with the 1906 san francisco earthquake indicates that uplift on the order of 10 15 cm occurred in the block northeast of the san andreas fault since 1906 5 6 cm of regional subsidence has occurred in that block one implication of our model is that the transfer of slip from the san andreas fault to a fault 5 km to the east the golden gate fault is not required for the area offshore of san francisco to be in extension this has implications for both the deposition of thick pliocene pleistocene sediments the merced formation observed east of the san andreas fault and the age of the peninsula segment of the san andreas fault

stressing of locked patches along a creeping fault an analysis is presented of the stressing of locked patches along a fault zone which is creeping elsewhere the model consists of strike slip faulted elastic lithospheric plates loaded in a manner equivalent to imposition of a remotely uniform tectonic shear stress the fault zone has an inhomogeneous strength distribution both depth wise and along strike it is modeled as being composed of locked patches and freely slipping parts treated as cracks the solution is given with the use of the line spring model which analyzes the problem by thickness averaged plane stress theory for lithospheric plates which slip along a discontinuity cut at the plate boundary as a boundary condition thickness averaged stress and slip at each local section along the cut was related to one another by the result of an antiplane strain analysis of slip for the crack or crack pair which describes the slipping and locked depth ranges at that section the analysis indicates that the slip distribution along creeping parts of the fault as well as the stress distribution along locked patches depends strongly on the geometry of these zones the model is used to examine stress concentrations associated with a slip deficient seismic gap along strike and to study the effect of local irregularities in the margin of a locked region it is also used to simulate slip and stressing processes associated with the creeping portion of the san andreas fault in central california between the presently locked zones of the great 1906 and 1857 ruptures and to constrain the nature of an apparently locked zone at the southeastern end which ruptures in characteristic parkfield earthquakes near fault creep and broadscale displacement data along the fault since the 1966 parkfield earthquake and inferences from seismicity distributions are used limitations of the modeling procedure at short spatial wavelengths prohibit an accurate description of the parkfield locked patch but results suggest that it may be localized and occupy a small fraction of area of the normal seismogenic zone an effective remote stressing rate of order 0 3 10 6 shear modulus yr is inferred together with a less well constrained 30 to 40 km lithospheric thickness for consistency with the displacement data results enable estimates of stress accumulation along the locked 1857 rupture zone and the build up of fracture energy release capability of order 107 j m2 in 150 yr at its lower margin

stress sensitivity of fault seismicity a comparison between limited offset oblique and major strike slip faults we present a new three dimensional inventory of the southern san francisco bay area faults and use it to calculate stress applied principally by the 1989 m 7 1 loma prieta earthquake and to compare fault seismicity rates before and after 1989 the major high angle right lateral faults exhibit a different response to the stress change than do minor oblique right lateral thrust faults seismicity on oblique slip faults in the southern santa clara valley thrust belt increased where the faults were unclamped the strong dependence of seismicity change on normal stress change implies a high coefficient of static friction in contrast we observe that faults with significant offset 50 100 km behave differently microseismicity on the hayward fault diminished where right lateral shear stress was reduced and where it was unclamped by the loma prieta earthquake we observe a similar response on the san andreas fault zone in southern california after the landers earthquake sequence additionally the offshore san gregorio fault shows a seismicity rate increase where right lateral oblique shear stress was increased by the loma prieta earthquake despite also being clamped by it these responses are consistent with either a low coefficient of static friction or high pore fluid pressures within the fault zones we can explain the different behavior of the two styles of faults if those with large cumulative offset become impermeable through gouge buildup coseismically pressurized pore fluids could be trapped and negate imposed normal stress changes whereas in more limited offset faults fluids could rapidly escape the difference in behavior between minor and major faults may explain why frictional failure criteria that apply intermediate coefficients of static friction can be effective in describing the broad distributions of aftershocks that follow large earthquakes since many of these events occur both inside and outside major fault zones

three dimensional normal faulting models of the valles marineris mars and geodynamic implications inversion of martian topography using a three dimensional boundary element model permits revised estimates of fault dip angle depth of faulting paleogeothermal gradient and extensional strain in the valles marineris region the major normal faults dip at 40 55 to depths of 60 75 km with comparatively small footwall uplifts 500 m for throws to 10 km implying that paleogeothermal gradients during faulting were 10 k km 1 or less assuming relatively rapid strain rates appropriate to the extending martian crust accumulation of dip slip offsets along the main normal faults likely was associated with anticlinal flexure and deformation of preexisting strata within the troughs the magnitudes of the predicted vertical and horizontal displacements outside the troughs as well as extensional strains are spatially variable depending on both cross and along strike position relative to the troughs the predicted displacements and strains attain maximum values at the troughs and decay to background values at cross strike distances of 250 km corresponding to 3 4 times the depth of faulting with an average province wide strain of 4 15 by implication the aggregate strain field surrounding tharsis is a highly variable spatial and temporal composite of the inhomogeneous strain fields each associated and scaling in size with an individual graben from the smallest structures through the largest valles marineris troughs

faulting of lemnos island a mirror of faulting of the north aegean trough northern greece abstract a detailed analysis of the faulting on lemnos island since the miocene was achieved by grouping the recorded meso scale faults and their slickenlines into different groups replacing them with mean representative fault planes mrfp and lineations mrfl and finding the stress tensors driven to the activation of the different mrfp mrfl pairs the oldest recognized fault activity followed the late oligocene n s contraction and uplift d1 event as the latter is implied by e w to ene wsw buckle folds in the upper eocene lower oligocene molasse type sediments it was an early middle miocene strike slip to transpression deformation d2 event associated with n s contraction that also affected the lower miocene volcanogenic rocks the next fault activity was dated in middle late miocene and caused a wnw ese extension d3 event this deformation suggests that a during the late eocene oligocene the north aegean trough was developed as a fore arc basin between the rhodopian magmatic arc and its subduction accretion complex to the south and b during the late oligocene middle miocene the north aegean trough as the whole hellenic hinterland was subjected to the transpression s l deformation and lateral extrusion because of the late collisional processes between apulia and eurasia plates and the retreating of the hellenic orogen from late miocene to pliocene a ne sw extension to right lateral transtension d4 event dominated the north aegean trough indicating that the retreating of the hellenic orogen was completely balanced by a back arc extension the deformation facilitates the westward propagation of the north anatolian fault into the north aegean sea since the early pleistocene the deformation of the north aegean trough is related to a nne ssw back arc extension d5 of the present day hellenic subduction zone and the dominant ene wsw to e w faults along the north aegean trough function as normal to oblique right lateral normal faults

cosmogenic 3he concentrations in ancient flood deposits from the coombs hills northern dry valleys east antarctica interpreting exposure ages and erosion rates abstract in situ produced cosmogenic 3 he analyses provide independent support for the model of a stable hyper arid polar climate persisting in east antarctica since the mid miocene and provide quantitative constraints on long term rates of erosion within the dry valleys in the coombs hills area a series of cobble size boulders form mega ripples with wavelengths of approximately 50 m their topographic position and association with features characteristic of scabland such as stripped corrugated bedrock surfaces indicate the boulders were deposited by subglacial floodwaters such outburst flooding could only have occurred during overriding of the northern dry valleys by a greatly expanded east antarctic ice sheet timing of the overriding episode has been previously assigned to 14 8 to 13 6 ma by correlation with volcanic ash deposits dated by 40 ar 39 ar in the asgard range of the dry valleys cosmogenic 3 he concentrations in clinopyroxene from ferrar dolerite boulders are consistent with 8 6 to 10 4 ma exposure calculated using scaling factors appropriate for antarctica and assuming zero erosion these are among the oldest surface exposure dates yet measured on earth but are not however consistent with the 40 ar 39 ar chronology used to define the age of the landscape due to unconstrained levels of erosion erosion rates of 0 03 0 06 m ma 1 are necessary to have produced the measured boulder exposure age if they were deposited at 14 8 ma these are less than half the steady state erosion rate derived from cosmogenic 3 he in the nearby bedrock surfaces 0 17 m ma 1 and testify to the extreme stability of the landscape

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

seismotectonics of the coast ranges in the vicinity of lake berryessa northern california tectonic deformation as manifested by seismicity and active faulting in the coast ranges in the vicinity of lake berryessa appears to be concentrated along the green valley cedar roughs fault trend and to a lesser degree the coast ranges sierran block crsb boundary zone the green valley cedar roughs fault trend appears to be a complex discontinuous or segmented shear zone consisting of multiple faults geomorphic evidence of quaternary displacement exists along many portions of the fault trend with holocene features and evidence for fault creep occurring along the green valley fault in contrast no historic earthquakes greater than ml 5 are known to have occurred along the fault trend earthquakes relocated in this study from 1970 to 1988 occur in the vicinity of mapped faults however the seismicity is discontinuous with a strong tendency to occur in clusters also superimposed on the general north northwest fault trend are north south epicentral trends not associated with any mapped faults focal mechanisms suggest predominantly right lateral strike slip faulting on both north and north northwest trending planes the existence of reverse and normal focal mechanisms also attests to the complex faulting occurring along the green valley cedar roughs trend in general the geologic and seismogenic characteristics are consistent with the hypothesis that the green valley cedar roughs fault trend represents young faulting possibly related to a newly developing plate boundary east of the san andreas fault earthquakes are confined to the top 12 km of the upper crust seismicity within the crsb boundary zone since 1970 has been generally confined to a sequence in 1978 near madison with a mainshock of ml 4 2 thrust reverse faulting appears to be the source of this activity consistent with the seismogenic processes observed along other segments of the boundary zone only one earthquake has occurred since 1970 in the epicentral area of the 1892 ml 6 vacaville winters earthquake the state of stress in the lake berryessa region as exhibited by focal mechanisms appears to be characterized by ne directed tectonic compression as observed elsewhere away from the san andreas fault system in northern and central california

dating of sirius group tillites in the antarctic dry valleys with cosmogenic3he and21ne we measured in situ produced cosmogenic3he and21ne in boulder and bedrock surface samples in and near sirius group tillites from two localities in southern victoria land antarctica pyroxene quantitatively retains cosmogenic3he for millions of years the deposit at mt fleming has a minimum exposure age of 6 5 ma while 6 ma is a probable minimum age for the table mountain tillite these lower limits not taking into account erosion of the sampled surfaces are based on the samples with the highest concentrations of cosmogenic noble gases and currently accepted production rates since the plagioclase bearing mt fleming samples almost certainly lost part of their cosmogenic ne the minimum exposure age at this location is presumably even 50 higher than the stated value the concentrations of cosmogenic ne in our samples constrain uplift rates of the transantarctic mountains to 170 m ma these data contradict the hypothesis of a collapse of the east antarctic ice sheet in the pliocene coupled with high uplift rates and support the model of a stable ice sheet since the middle miocene

equatorial pacific productivity and dust flux during the mid pleistocene climate transition received 6 may 2005 revised 30 august 2005 accepted 19 september 2005 published 17 december 2005 1 we present a helium isotope record for core tt013 114pc from the central equatorial pacific 140 w 4 n 4432 m water depth spanning a period of 1 million years we focus on the time interval from 560 to 800 kyr largely coinciding with the mid pleistocene climate transition mpt when the dominant period of the earth s climate variability shifted from 41 kyr to 100 kyr the terrigenous 4 he concentrations from our study correlate very well with published titanium concentrations in this core strongly supporting the use of terrigenous 4 he as a monitor of continental dust normalizing titanium and terrigenous 4 he concentrations to 3 he suggests that the dust supply during the mpt was approximately 30 lower compared to the subsequent period 560 100 kyr the 3 he normalized barium aluminum and phosphorus concentrations trace elements with a predominantly biogenic source in these sediments are relatively constant this is in contrast to previous studies that reported an apparent rise of titanium normalized productivity proxies rather than a significant increase in productivity during the mpt we conclude that the dust flux to the central equatorial pacific was reduced and that the export productivity was approximately constant during this period of climate reorganization

a new extraction technique and production rate estimate for in situ cosmogenic 14c in quartz the potential of in situ cosmogenic 14c in situ 14c for surficial process studies is widely recognized yet a reliable means of isolating it has proved difficult to develop we present here a new method for extracting in situ 14c from quartz that overcomes difficulties encountered with earlier techniques yielding more reliable production rate estimates comparison of 14c thermal release patterns from surficial and deeply shielded quartz samples lifton 1997 demonstrated that contaminant 14c is released at or below 500 c and that 14c released between 500 and 1500 c is essentially all in situ produced the new technique builds on this key result using resistance heating of samples in the presence of a lithium metaborate libo2 flux and collection of all evolved carbon as co2 between 500 c and 1100 to 1200 c our improved method has four distinct advantages over other extraction methods 1 we can identify and quantitatively eliminate atmospheric organic 14c contamination 2 we can identify the in situ 14c component unambiguously without assumptions of 14co 14co2 production proportions within the rock or equilibria on extraction 3 in situ 14c is reliably extracted from quartz at lower temperatures and in less time than earlier methods and 4 blank 14c levels are consistently low 2 3 0 1 105 14c atoms 1 our new extraction procedures should thus enable researchers to use in situ 14c in diverse applications without reservation we developed our new procedures using samples of wave cut quartzite benches from the well dated bonneville 17 4 0 3 cal ky and provo 16 8 0 3 cal ky shorelines of pleistocene lake bonneville utah and from underlying deeply shielded locations in situ 14c was extracted from quartz separated from 2 bonneville shoreline samples 6 aliquots and 1 provo shoreline sample 2 aliquots results demonstrate that our new procedures can effectively isolate the in situ 14c fraction with replicate analytical precision better than 2 1 n 5 while remaining consistent with earlier results this level of precision and accuracy is comparable to or exceeds those currently obtainable with in situ cosmogenic 10be 26al 3he 21ne and 36cl resulting weighted mean in situ 14c site production rates for the bonneville and provo shorelines are 52 9 1 7 14c atoms g sio2 y and 48 7 2 8 14c atoms g sio2 y 1 respectively consistent with earlier production rate estimates current and previously published in situ 14c site production rate estimates were then scaled to sea level and high geomagnetic latitude using the latitude altitude scaling models of lal 1991 and dunai 2000 results indicate that both models yield sea level high latitude production rates consistent with independent estimates our new in situ 14c data yield integrated late quaternary production rate estimates at sea level and high latitude of 15 1 0 5 14c atoms g sio2 y using the lal 1991 model and 15 8 0 5 14c atoms g sio2 y with that of dunai 2000 until significant uncertainties in these models are addressed however we prefer the value from the widely used lal 1991 model as our best estimate of the integrated late quaternary production rate for in situ 14c

corrections to heat flow and energetics of the san andreas fault zone and some additional comments on the relation between fault friction and observed heat flow

cosmogenic 53mn in the main fragment of the norton county aubrite abstract cosmogenic 53mn was determined for the first time in an aubrite a very fe poor achondrite six samples of the main fragment of norton county were selected near locations where preatmospheric depths of 11 21 cm had been determined from cosmic ray tracks the measured 53mn activities ranged from 414 to 552 dpm kg fe and increased rapidly with depth production rates of 53mn calculated with the lahet code system agree with the measured activities and show that the radius of norton county 65 cm was near that for maximum production rates no special irradiation conditions are needed to explain cosmogenic nuclides in norton county

the kinematic paradox of the san andreas fault the pacific north america plate boundary along the san andreas fault system is notoriously a right lateral transpressive margin where both almost pure thrust and strike slip tectonics take place the pacific plate travels wnw forming an angle of about 25 with the boundary since the pacific is moving wnw faster than north america right lateral transtension should result along the san andreas system north america in turn travels westward obliquely to the boundary and a left lateral transpressive component would be expected along the same margin therefore the right lateral transpression of the san andreas system can be partitioned into i a sinistral transpression along the southwestern margin of the north america plate obliquely overriding ii a faster right lateral transtension occurring along the transfer margin of the pacific plate between the east pacific rise in the california gulf and the gorda ridge to the north west this is due to the oblique trend of the pacific and north america plate margins with respect to their motion in a absolute reference frame the geodynamics of california is marked by a unique setting in which there is a special subduction where in contrast with classic subduction zones the footwall of the subduction plane is obliquely diverging from the hanging wall in an e w section while it is converging at slower rates in a ne sw direction the extensional e w component is absorbed into the basin and range rifting whereas the compressive ne sw component is mainly expressed in the coast ranges and california offshore the compression perpendicular to the san andreas is then not intrinsic in the strike slip movement but it is rather an independent tectonic factor therefore the san andreas system cannot be considered as an archetype of a pure strike slip fault

transform faults orthogonal to rifts insights from fully gravitational physical experiments abstract on the one hand it is not yet understood why transform faults commonly form orthogonal to rifts because this cannot be explained by simple theories of fracture mechanics on the other hand rock experimental data have shown that a very strong layer elastic core should exist between brittle and ductile layers of the lithosphere therefore we investigated the influence of an elastic core on the generation of a transform fault rift orthogonal configuration we used fully gravitational scaled physical experiments of brittle viscous models and the results indicate that the elastic core and localized heat flow can be responsible for the observed common pattern of transforms orthogonal to rifts

cenozoic marine sedimentation and ice volume variation on the east antarctic craton recycled cretaceous and cenozoic marine microfossils have been recovered from samples of the pliocene sinus formation samples were collected from outcrops in the reedy beardmore and ferrar glacier areas of the transantarctic mountains between lat 77 and 86 s the glaciogene sediments contained diatoms foraminifera calcareous nannoplankton silicoflagellates radiolarians sponge spicules palynomorphs and ostracodes of late cretaceous paleocene eocene late oligocene late miocene and pliocene age this suggests the presence of open marine basins on the east antarctic craton during late mesozoic and cenozoic time the apparent absence of early oligocene and early through middle and earliest late miocene assemblages suggests either that marine regression exposed the basin floors or that ice filled the basins during these times the high elevation setting of sirius formation outcrops suggests one of two hypotheses for their origin 1 they are in situ pliocene glaciomarine deposits that were uplifted 1 750 2 500 m with the transantarctic mountains to their present elevation 2 the sirius formation deposits are a mixture of derived sediments stripped from sub ice intracratonic basins and subsequently redeposited by ice flowing up the inland slope of the transantarctic mountains we favor the second hypothesis with transport to sites sometime within the past 3 m y

revealing the pace of river landscape evolution during the quaternary recent developments in numerical dating methods during the last twenty years several technical developments have considerably intensified the use of numerical dating methods for the quaternary the study of fluvial archives has greatly benefited from these enhancements opening new dating horizons for a range of archives at distinct time scales and thereby providing new insights into previously unanswered questions in this contribution we separately present the state of the art of five numerical dating methods that are frequently used in the fluvial context radiocarbon luminescence electron spin resonance esr 230th u and terrestrial cosmogenic nuclides tcn dating we focus on the major recent developments for each technique that are most relevant for new dating applications in diverse fluvial environments and on explaining these for non specialists therefore essential information and precautions about sampling strategies in the field and or laboratory procedures are provided for each method new and important implications for chronological reconstructions of quaternary fluvial landscapes are discussed and where necessary exemplified by key case studies a clear statement of the current technical limitations of these methods is included and forthcoming developments which might possibly open new horizons for dating fluvial archives in the near future are summarised

fault plane solutions for the 1984 morgan hill california earthquake sequence evidence for the state of stress on the calaveras fault fault plane solutions for 946 aftershocks of the april 24 1984 morgan hill california m6 2 earthquake reveal a pattern of complex faulting within a 10 km wide zone surrounding the calaveras fault the fault plane solutions fall into three groups strike slip mechanisms located along the calaveras fault with north northwest striking dextral slip planes nearly parallel to the fault strike slip mechanisms located northeast of the calaveras fault with north striking dextral slip planes and reverse mechanisms located southwest of the calaveras fault with northeast or southwest dipping slip planes the average azimuth of p axes for aftershocks located on the calaveras fault is n10 1 e in contrast the average azimuths of p axes for aftershocks northeast and southwest of the calaveras fault are n49 7 e and n37 3 e respectively by assuming that the earthquakes occur on preexisting cracks in response to a uniformly oriented regional stress field we are able to infer from the observed average slip directions an orientation for 1 the axis of maximum horizontal compression 63 80 from the local strike of the calaveras fault such a high angle orientation of 1 is incompatible with the classical andersonian model of strike slip faulting but is consistent with an alternate model in which a weak calaveras fault is driven by plate motion slightly convergent with respect to the san andreas fault system in central california if this inferred high angle uniform stress field is combined with the changes in the static stress field calculated with elastic dislocation theory for the 1984 morgan hill main shock the resulting stress field predicts most of the observed spatial pattern of the aftershocks and their fault plane solutions the orientations of the inferred aftershock slip planes are generally consstent with the fracture orientations predicted by the coulomb failure criterion areas with predicted increases in the coulomb failure function of only a few tenths of a megapascal correspond to areas of intense aftershock activity the invariance of focal mechanisms in the vicinity of the south end of the main shock rupture before and after the main shock suggests that the magnitude of compressive stress in this area is at least 10 mpa in addition to explaining a preponderance of the seismic observations of the aftershock sequence the presence of a predominantly fault normal compressive regional stress field accounts for the contemporary development of folds reverse faults and topographic relief observed throughout the central california coast ranges

a comparative study of accumulation rates derived by he and th isotope analysis of marine sediments abstract we present a detailed down core analysis of helium isotope ratios and concentrations for bulk sediments from the central equatorial pacific that span the last two glacial interglacial cycles measured 3 he 4 he ratios range from 1 0 10 5 to 2 1 10 4 or 7 4 to 149 times the atmospheric ratio the 3he from interplanetary dust particles idps constitutes virtually all of the 3he measured within the sediment because carbonate accumulation rates are high in the equatorial pacific the measured 3he concentrations are lower than have been measured elsewhere and range from 4 7 10 13 to 3 0 10 12 cm3stp g 1 if the cosmic dust 3he flux is constant with time sediment mass accumulation rates can be determined from the 3he concentration in sediments the excess 230th technique is an entirely independent method for calculating sediment mass accumulation rates because its source is in situ decay of 234u in seawater to first order initial excess 230th activities correlate with 3he concentrations within this core based on the 230th results we estimate the 3he flux to the earth s surface as 9 6 2 0 10 16 cm3stp cm 2 a 1 if this flux has remained constant over extended periods of time it can be used to determine sediment accumulation rates beyond the 230th range 300 000 yr

state of stress and crustal deformation along weak transform faults the state of stress and style of crustal deformation along weak transform plate boundaries is discussed in the context of available data and simple analytical models appreciable evidence indicates that while the frictional strength of the upper crust is high in general accordance with simple faulting theory and laboratory derived coefficients of friction the frictional resistance to motion along transform plate boundaries is extremely low these conditions require that horizontal principal stresses must be oriented approximately parallel and perpendicular to the transform fault zone to minimize the shear stresses acting parallel to the transform along plate margins that must accommodate relative convergence transpressive margins a pattern of near fault normal compression and fault normal crustal shortening is expected along divergent plate margins transtensional margins extension is expected to occur perpendicular to the transform as the direction of minimum horizontal principal stress is expected to be nearly perpendicular to it these patterns of stress and deformation can be observed along a number of transform faults around the world

length displacement scaling and fault growth abstract following an earthquake in a fault zone commonly the co seismic rupture length and the slip are measured similarly in a structural analysis of major faults the total fault length and displacement are measured when possible it is well known that typical rupture length slip ratios are generally orders of magnitude larger than typical fault length displacement ratios so far however most of the measured co seismic ruptures and faults have been from different areas and commonly hosted by rocks of widely different mechanical properties which have strong effects on these ratios here we present new results on length displacement ratios from 7 fault zones in holocene lava flows on the flanks of the volcano etna italy as well as 10 co seismic rupture length slips and compare them with fault data from iceland the displacement and slip data from etna are mostly from the same fault zones and hosted by rocks with largely the same mechanical properties for the co seismic ruptures the average length is 3657 m the average slip 0 31 m and the average length slip ratio 19 595 for the faults the average length is 6341 m the average displacement 73 m and the average length displacement ratio 130 thus the average rupture slip ratio is about 150 times larger than the average length displacement ratio we propose a model where the differences between the length slip and the length displacement ratios can be partly explained by the dynamic young s modulus of a fault zone being 101 2 times greater than its static modulus in this model the dynamic modulus controls the length slip ratios whereas the static modulus controls the length displacement ratio we suggest that the common aseismic slip in fault zones is partly related to adjustment of the short term seismogenic length slip ratios to the long term length displacement ratios fault displacement is here regarded as analogous to plastic flow in which case the long term displacement can be very large so long as sufficient shear stress concentrates in the fault

terrestrial cosmogenic argon cosmogenically enriched 38 ar 36 ar ratios are detectable in ca bearing terrestrial samples and are resolvable from a uniform background value of 0 18826 0 00018 for materials lacking nucleogenic or kinetic mass fractionation effects relationships between 38 ar 36 ar and cosmogenic 3he in apatite and fluorite are systematic but difficult to reconcile with current estimates of terrestrial production rates for 38 ar and 36 ar from ca suggesting that production from cl and or other targets is significant or that the estimates are inaccurate in either case the effects of time varying production of 36 ar from decay of cosmogenic 36 cl produced from ca k or cl on the 38 ar 36 ar ratio must be accounted for

the ksar ghilane 002 shergottite the 100th registered martian meteorite fragment we report on the discovery of a new shergottite from tunisia ksar ghilane kg 002 this single stone weighing 538 g is a coarse grained basaltic shergottite mainly composed of maskelynitized plagioclase approximately 52 vol and pyroxene approximately 37 vol it also contains fe rich olivine approximately 4 5 vol large ca phosphates including both merrillites and cl apatites approximately 3 4 vol minor amounts of silica or sio 2 normative k rich glass pyrrhotite ti magnetite ilmenite and accessory baddeleyite the largest crystals of pyroxene and plagioclase reach sizes of approximately 4 to 5 mm pyroxenes fs 26 96 en 5 50 wo 2 41 they typically range from cores of about fs 29 en 41 wo 30 to rims of about fs 68 en 14 wo 17 maskelynite is ab 41 49 an 39 58 or 1 7 in composition but some can be as anorthitic as an 93 olivine fa 91 96 occurs mainly within symplectitic intergrowths in paragenesis with ilmenite or at neighboring areas of symplectites kg 002 is heavily shocked s5 as indicated by mosaic extinction of pyroxenes maskelynitized plagioclase the occurrence of localized shock melt glass pockets and low radiogenic he concentration oxygen isotopes confirm that it is a normal member of the snc suite kg 002 is slightly depleted in lree and shows a positive eu anomaly providing evidence for complex magma genesis and mantle processes on mars noble gases with a composition thought to be characteristic for martian interior is a dominant component measurements of 10 be 26 al and 53 mn and comparison with monte carlo calculations of production rates indicate that kg 002 has been exposed to cosmic rays most likely as a single meteoroid body of 35 65 cm radius kg 002 strongly resembles los angeles and nwa 2800 basaltic shergottites in element composition petrography and mineral chemistry suggesting a possible launch pairing the similar cre ages of kg 002 and los angeles may suggest an ejection event at approximately 3 0 ma

physical models of rifting and transform faulting due to ridge push in a wedge shaped oceanic lithosphere by scaled physical modelling we have investigated the mechanical response to gravitational forces in an oceanic lithosphere overlying a less dense asthenosphere in the models an upper wedge shaped layer of sand represented an oceanic lithosphere 0 35 ma old with a half spreading velocity of 3 cm yr and a lower layer of polydimethylsiloxane pdms mixed with dense wolframite powder represented the asthenosphere in the models as in nature isostatic compensation resulted in uplift of ridges and subsidence on their flanks the resulting relief was responsible for ridge push we tested two main configurations straight ridges and offset ridges in all the models ridge push was sufficient to cause plate motion underlying advection and symmetrical rifting at the ridge axis there was no need to impose plate motions through external pistons and motors in models of straight ridges the style of normal faults in the axial rift zone depended on the local thickness of the brittle sand layer for thick layers normal faults rafted out from the active zone of rifting creating a fossil topography of tilted blocks between faults dipping toward the ridge in a model of an offset ridge with thin lithosphere at the ridge crest and no embedded weakness ridge push was responsible for a short transform fault linking en echelon rifts in a similar model but with thick lithosphere an oblique rift formed at about 20 to the offset trace we conclude that ridge push was not adequate to create an ideal transform fault in a model of an offset ridge with an embedded thin vertical layer of pure pdms at 90 to the ridge transform motion concentrated along this weak layer and the resulting structural style was very similar to that in nature on the basis of these results we infer that in nature 1 ridge push can indeed drive plate motion and 2 ridge push can drive strike slip motion on transform faults provided that these are weaker than the adjacent oceanic lithosphere and that they form early in the history of spreading

fracturing and hydrothermal alteration in normal fault zones large normal fault zones are characterized by intense fracturing and hydrothermal alteration displacement is localized in a slip zone of cataclasite breccia and phyllonite surrounding corrugated and striated fault surfaces slip zone rock grades into fractured but less comminuted and hydrothermally altered rock in the transition zone which in turn grades abruptly into the wall rock fracturing and fluid flow is episodic because permeability generated during earthquakes is destroyed by hydrothermal processes during the time between earthquakes

normal faulting and block tilting in lofoten and vesteralen constrained by apatite fission track data abstract mesozoic and possibly younger normal faulting and block tilting in the lofoten vesteralen archipelago can be constrained by apatite fission track data previous studies have documented the polyphase structural evolution of the archipelago based on field data and onshore offshore correlations our new aft data document vertical movements on a more regional scale with kilometer scale offset on some faults below sortlandsundet the hadselfjord fault zone forms the eastern limit of the sortlandsundet half graben and our data reveal latest cretaceous and younger half dome shaped uplift of the footwall east of the western lofoten border fault on vestvagoya southeastward tilting is indicated both by the pattern of aft ages ranging from 81 7 ma to in the nw to 167 16 ma in the se and tilted paleosurfaces aft ages immediately east of sortlandsundet 72 5 ma and on vestvagoya 81 7 ma are among the youngest ones found on the norwegian mainland the present day landscape of the lofoten vesteralen archipelago does not represent a re exposed and glacially modified mesozoic landscape although most of our data indicate cooling to below 60 c took place during the mesozoic the youngest footwall ages are hard to explain without alluding to cenozoic normal faulting a cenozoic structural control on the landscape can thus be inferred for lofoten and vesteralen

scales of structure in the lithosphere images of processes advances in deep seismic sounding observations of the lithosphere on continents allow to identify various scales of heterogeneities in the crust mantle transition in this paper a fresh look is taken at the dynamic properties of seismic waves propagating through this zone ranging from the lower crust well into the lithospheric mantle as observed in near vertical reflection and wide angle refraction experiments the abrupt termination of near vertical reflections at the moho and the coincident presence of a strong coda accompanying the supercritical pmp reflection phase in record sections of continental seismic wide angle refraction experiments reveal a new property of the crust mantle boundary the moho forms a sandwiched mix of crust mantle material between lower crust and topmost mantle in combination with a step in mean velocity a significant change in scale of the structural dimensions and of velocity variance this view is strongly supported by the recognition of the subcrustal lithosphere as a high frequency 5 10 hz scattering wave guide for long distance propagation of the pn phase over more than 3000 km detected during seismic experiments in russia using peaceful nuclear explosions evidence is brought forward that this high frequency wave guide is likely to be present also in the phanerozoic upper mantle of western europe it is furthermore hypothesized that the global propagation of teleseismic sn or corresponding pn waves observed on continents and oceans combined with the recording of strong pmp codas for wave paths on continents points to a world wide change of scale of structure and velocities near the moho a change in scale of heterogeneities under almost all physical and chemical conditions indicates possibly a differential horizontal movement or decoupling of crust and upper mantle at the level of the moho the global presence of the scattering pn wave guide with a preferred horizontal elongation is hypothetically described as a seismic image of processes in the lithospheric mantle locations of present day tectonic activity where horizontal decoupling between crust and mantle are likely to occur e g subduction at continent continent collision zones change of tectonic regimes and continental transform faults are discussed

correlation between the geologic record and computed plate motions for central california we have attempted to correlate geologic events in part of central california with computed motions of oceanic plates relative to cratonic north america three composite terranes are considered 1 the sierra nevada 2 the coast ranges northeast of the san andreas fault and 3 the salinian block in the sierra nevada jurassic plutonism ending about 147 m y b p and cretaceous plutonism 120 80 m y b p correlate with deduced farallon north america fa na convergence as does the nevadan orogeny 158 153 m y b p however the gap in plutonism 146 121 m y b p outlasted by far an apparent minimum in convergence 145 135 m y b p magmatism ceased in the sierra and uplift slowed during the laramide orogeny 75 45 m y b p when tectonic activity had moved east to the site of the rockies presumably because of a low angle of subduction but volcanisin resumed in the sierra about 33 m y b p indicating that a steeper angle was reestablished apparently east west compression slackened when the pacific north america pa na transform regime developed basalt was erupted in the southern sierra 12 3 m y b p and during this same time the range started its most rapid rise as a normal fault bounded block the coast ranges north east of the san andreas fault are underlain by the franciscan complex the older parts of the complex were probably assembled in a subduction zone far south of their present location but we believe that the same fa na convergence that was cited for sierran plutonism was responsible despite the disparity in latitude as the farallon plate was probably in contact with most of the western margin of north and south america a perceived strong pulse in convergence 100 85 m y b p evidently produced the coast range thrust which on geologic grounds alone most likely originated between 96 and 88 m y b p some of the northwesterly transport of franciscan rocks relative to north america is ascribed to oblique kula north america ku na convergence 85 43 m y b p during which period the coast range thrust was revived about 60 m y b p the farallon plate again influenced the california margin after the kuia plate had moved northward and fa na convergence caused the accretion of the coastal belt and king range terranes of the neo franciscan the advent of pacific north america pa na interaction was marked by local volcanism which advanced from the southeast toward the northwest in consonance with the passage of the mendocino triple junction dickinson and snyder 1978 in the salinian block plutons 107 82 m y old can be ascribed to the same fa na convergence as the contemporary cretaceous plutons of the sierra nevada although the salinian intrusives originated much farther south paleomagnetic results champion et al 1980 from the salinian related pigeon point formation 75 71 m y in age suggest about 2500 km of poleward essentially coastwise movement this postulated large movement during part or all of the last 75 m y poses the greatest dilemma in our study the kuia is the only known plate whose computed obliquely convergent motion is appropriate for the late cretaceous paleogene part of the migration and even this motion is inadequate for the estimated transport within the most likely time window furthermore the farallon plate seems ideally suited for the laramide orogeny 70 45 m y b p both plates could not have been in contact with the same part of the margin at the same time oblique convergence of the kula plate could possibly have caused large coastwise transport and also the laramide orogeny the coast ranges northeast of the san andreas fault and the salinian block both experienced strike slip faulting and en echelon folding correlative with pa na interaction indicating that they were close together by neogene time faulting and folding have been particularly marked since the inception of spreading in the gulf of california and attachment of westernmost california to the pacific plate about 4 m y b p the rise of individual coastal ranges commencing 3 1 m y b p may possibly have been caused by slight convergence which appears in computed pa na relative motions from 5 m y b p to the present

a theory of intraplate tectonics the theory of plate tectonics assumes a rigid behavior of tectonic plates and therefore fails to account for observed intraplate deformation a new theory of intraplate tectonics is developed to calculate the first order intraplate deformation induced by horizontal displacement of deformable plate boundaries it is based on simple assumptions that link the well established directions of relative plate motion to the displacement and deformation fields within a plate interior adjacent to three types of deformable plate boundaries inward outward and tangential displaced boundaries the theory predicts the direction of intraplate displacement displacement rate strain and stress fields in terms of small circles great circles and 45 loxodromes around the pole of rotation of two adjacent plates the 45 loxodromes are two orthogonal sets of directions clockwise and counterclockwise that intersect both small and great circles at 45 the principal axis of the maximum horizontal stress follows small circles for inward displaced boundaries great circles for out ward displaced boundaries and loxodromes for tangential displaced boundaries the theoretical predictions are systematically compared with more than 4000 reliable observed directions of maximum horizontal stress provided by the world stress map project zoback 1992 the theory indicates that the first order intraplate deformation is predominantly induced by horizontal forces acting on plate boundaries and by buoyancy forces that arise from lateral density variations between mid ocean ridges and plate interiors ridge push the simplicity of the predictions and their good agreement with observations suggests that intraplate deformation should be investigated in the pole of rotation spherical coordinate system

in situ cosmogenic 10be and 21ne in sanidine and in situ cosmogenic 3he in fe ti oxide minerals abstract we report concentrations of in situ cosmogenic 10 be and 21 ne from coexisting quartz and sanidine separates and of cosmogenic 3 he in coexisting fe ti oxide minerals from ignimbritic successions of northern chile oxaya and lauca ignimbrites new mineral isotope pairs such as sanidine and fe ti oxide minerals are helpful in quantitative geomorphology for geological settings where the lithology lacks the commonly used minerals quartz pyroxene or olivine production rates in sanidine and fe ti oxide minerals were determined by normalizing nuclide concentrations to established production rates in quartz the experimentally determined production rates are compared to model production rates calculated with new cross sections for 3 he 21 ne and 10 be production from the individual target elements the mean experimental 21 ne production rate for five sanidine samples is 30 4 3 7 atoms g 1 yr 1 30 4 5 4 atoms g 1 yr 1 including the uncertainty of the 21 ne production rate in quartz p 21 ne qtz this is in excellent agreement with the modelled value of 28 3 atoms g 1 yr 1 which has an estimated uncertainty of 20 the 21 ne production rate in sanidine is thus about 50 higher than that in quartz the cosmogenic neon in sanidine is entirely released in low temperature steps 400 600 c and no signs of an interfering nucleogenic neon component were observed this is in stark contrast to quartz and makes sanidine an attractive mineral for terrestrial cosmogenic neon studies 3 he diffuses out of the sanidine structure preliminary results also indicate that sanidine is well suited for 10 be studies the mean experimental 10 be production rate from two sanidine samples is 4 45 0 38 atoms g 1 yr 1 4 45 0 42 atoms g 1 yr 1 including uncertainty of p 10 be qtz very close to the modelled value of 4 55 atoms g 1 yr 1 which has an estimated uncertainty of 20 we emphasize that 10 be and 21 ne production in sanidine is composition dependent therefore major element analyses should be carried out and production rates calculated on a sample by sample basis fe ti oxide minerals retain 3 he quantitatively experimentally derived production rates are in excellent agreement with new values derived from physical modelling the mean experimental 3 he production rate for five fe ti oxide minerals samples is 120 11 atoms g 1 yr 1 120 12 atoms g 1 yr 1 including the uncertainties of p 21 ne qtz compared to a mean modelled value of 124 atoms g 1 yr 1 which has an estimated uncertainty of 20 the variable chemical and structural composition of the solid solution lines of fe ti oxide minerals has little effect on the total 3 he production rate cosmogenic 21 ne is not produced in significant quantities in fe ti oxide minerals due to the absence of suitable target elements

continental fault structure and the shallow earthquake source plate boundaries in continental crust are generally less sharply defined than in the oceans with seismicity spread over broad areas interplate displacements appear to be largely accommodated by networks of major fault zones a simple 2 level model for these important structures accounts for the depth distribution of most continental earthquakes and for the observed range of faulting styles and associated rock deformation textures the model consists of a seismogenic frictional slip regime overlying quasi plastic mylonite belts wherein shearing is largely accommodated aseismically due mainly to the changing response of quartz to deformation with increasing temperature shear resistance increases with depth to a peak value in the vicinity of the frictiona1 quasi plastic transition and then decreases rapidly the depth to which microseismic activity extends appears inversely related to regional heat flow and can be satisfactorily modelled as the frictional quasi plastic transition for different geotherms using laboratory determined flow laws for quartz bearing rocks larger earthquake ruptures m 5 5 tend to nucleate near the base of the seismogenic regime in the region inferred to have the highest shear resistance and concentration of distortional strain energy consideration is also given to the depression of isotherms and seismic activity in regions of thrusting and to the question of the downward continuation of major fault zones through the lithosphere decoupling of the upper crust on flat lying shear zones may accompany higher level dip slip and perhaps in some circumstances strike slip faulting being favoured by above average continental heat flow and a high quartz content in the middle or deep crust the average level of deviatoric stress within the seismogenic regime remains an outstanding problem

numerical modeling of cenozoic stress patterns in the mid norwegian margin and the northern north sea numerical modeling of cenozoic stress patterns in the northern north sea and the mid norwegian margin is presented and the sense of potential slip along major fault planes belonging to the two areas is restored we assume that the main regional source of stresses is the atlantic ridge push as demonstrated by previous studies furthermore we also assume a nearly consistent nw se strike for the far field stress from continental breakup between greenland and norway earliest eocene to present day first we applied the commercial two dimensional distinct element method udec to simulate cenozoic stress and displacement patterns in the study area variations in rheology and major fault zones were introduced into the model the more trondelag fault complex and its inferred continuation into the shetland platform forms the major mechanical discontinuity in the model second we used the sortan method developed at the university of paris vi to predict the sense of potential slip along major fault planes the input for the sortan model was constrained by the geometry of the selected fault planes and local principal stress directions extracted from the udec modeling our results show that the more trondelag fault complex and its inferred continuation into the shetland platform act as a weak fault zone this fault zone divides the study area into two different stress provinces the continental margin and the northern north sea this result agrees well with the observed differences in cenozoic structural evolution of the two areas compressive structures are observed along the continental margin whereas relative tectonic quiescence characterizes the northern north sea during the tertiary the restored stress patterns in the northern north sea and the mid norwegian margin also agree well with the observed present day stress configuration our analysis demonstrates a method to reconstruct the sense of slip on major fault planes by combining two complementary numerical tools udec and sortan as a result it is demonstrated that oblique slip motions are mainly expected in particular strike slip and reverse dip slip faulting are simulated

microearthquake evidence for extension across the kane transform fault we report the results of a microearthquake experiment conducted in 1987 in the active transform portion of the kane fracture zone the kane is a slow slipping 25 mm yr large offset 150 km transform delineated by a pronounced transform valley the experiment site lies in the eastern half of the transform 40 km west of the eastern ridge transform intersection in a region of marked transform parallel topography the network consisted of 12 ocean bottom hydrophones and seismometers and extended for 25 km along the transform and 10 km in width spanning the transform valley and a narrow median ridge to the north hypocentral parameters were determined for 86 earthquakes epicenters within the network define a narrow transform parallel trend along the steep southern wall of the transform valley maximum focal depths increase from 6 to 9 km from east to west across the network significant activity was also detected outside the transform tectonized zone to the southeast this activity may be associated with the inside corner of the ridge transform intersection while to the north about 20 events occurred in 7 m y old crust in a region of ridge parallel bathymetry six focal mechanisms were obtained from p wave first motions for events within the transform valley the best constrained solutions for four earthquakes within the network show normal faulting on fault planes subparallel to the trend of the transform all the mechanisms indicate a tension axis perpendicular to the trend of the transform these results together with a significant historical record of large earthquakes near the experiment site lead us to conclude that the principal transform displacement zone was inactive during our experiment and that the activity we recorded is the result of extension in the adjacent lithosphere the observed focal mechanisms and the inference that the axis of least compressive stress is approximately perpendicular to the transform provide direct evidence that the transform fault is mechanically weak relative to the surrounding lithosphere potential sources of extension across the transform include thermal stress in the young oceanic lithosphere topographic loading a small component of plate divergence normal to the transform and northward motion of the asthenosphere relative to the surface plate

cosmogenic noble gas studies in the oldest landscape on earth surface exposure ages of the dry valleys antarctica extraordinarily high surface exposure ages have been determined for sirius group tillites of mt fleming and mt feather as well as at localities in the inner dry valleys using cosmogenic helium and neon ages of 10 ma at mt fleming 5 3 ma at mt feather and 6 5 ma at insel mountain are among the highest nominal exposure ages published so far these values are minimal ages as they are based on the assumption of zero erosion and uplift the mt feather sample independently confirms the pre pliocene age of the sirius group sediments in the dry valleys as previously determined at mt fleming the insel mountain samples provide evidence for a landscape formation of the inner dry valleys not later than late miocene time assuming conservatively low values of 2 5 cm ma 1 for erosion rate and 50 m ma 1 for uplift rate we infer that the sirius group tillites at mt fleming were deposited earlier than 20 ma ago this indicates that the overriding of the dry valleys block of the transantarctic mountains by the east antarctic ice sheet occurred not later than the early miocene maximum long term erosion rates in the inner dry valleys must be 15 cm ma 1 down to altitudes 1000 m since such low erosion rates require permanently cold and hyperarid conditions the response of antarctica to the pliocene warm climatic episode must have been small cosmogenic nuclide data from both the inner dry valleys and the sirius group sediment localities support the hypothesis of a stable east antarctic ice sheet since at least late miocene time implying that the climate of antarctica was decoupled from that of lower southern latitudes we present also new elemental 21ne production rates of p21 mg 196 atoms g 1 yr 1 and p21 al 55 atoms g 1 yr 1 at sea level and high geomagnetic latitude these figures are consistent with a 3he production rate of p3 110 atoms g 1 yr 1 similar to previously published values this consistency provides evidence that pyroxene is retentive for both helium and neon over at least 10 ma cosmogenic ne in quartz and pyroxene has a 22ne 21ne cos ratio of 1 266 0 040 and 1 159 0 040 respectively

recent stress field of the sinai subplate region abstract quaternary and recent tectonic stress data for the sinai subplate region have been compiled including focal mechanism solutions in situ stress measurements fault slip data and alignments of young volcanic feeders the average direction of maximum horizontal stress sh has been determined by means of a linear interpolation method the results indicate that the direction of the average maximum horizontal stress is closely parallel to the direction of the absolute motion of africa analysis of the orientation of principal stress directions suggests that a large part of the sinai subplate region is subjected to a strike slip regime characterized by an average maximum horizontal stress trend of nw 54 w for example out of 50 earthquake focal mechanism solutions 52 are pure strike slip and 10 are either normal or thrust fault with a remarkable strike slip component the remaining are pure normal fault 18 thrust fault 12 or undetermined 8 three different tectonic subprovinces can be delineated northwestern egypt the eastern mediterranean israel lebanon and northern syria are characterized by strike slip often with a remarkable thrust faulting component stress regime the northern red sea the sinai peninsula and a significant part of the arabian plate are characterized by strike slip often with a remarkable normal faulting component stress regime this is also the case for southwestern turkey

late miocene extension in coastal sonora mexico implications for the evolution of dextral shear in the proto gulf of california oblique rift abstract the timing kinematics and processes responsible for the rapid transition from subduction to oblique rifting and the localization of the pacific north america plate boundary in the gulf of california are not well understood well exposed volcanic rocks deposited between 15 and 10 ma in the sierra bacha coastal sonora mexico preserve a record of late miocene deformation on the eastern rifted margin of the gulf of california and offer new insights into the timing and kinematic evolution of oblique rifting detailed geologic mapping fault kinematic analysis u pb and 40ar 39ar geochronology and paleomagnetic data reveal that the 2 km thick composite volcanic section is cut by a series of southwest dipping domino style normal faults and uniformly tilted down to the northeast palinspastic cross section restoration suggests that the region experienced ca 55 60 northeast southwest directed extension between 11 7 and 10 9 ma fault kinematic data reflect relatively minor dextral transtension either following or during the later stages of extension paleomagnetic results indicating modest clockwise vertical axis block rotation suggest that dextral shear was concentrated in the southwest of the study area near the modern coastline these results support an emerging model in which dextral strain was not ubiquitous across sonora and did not initiate immediately following the 12 5 ma transition from subduction to oblique rifting instead strain east of the baja california microplate at this latitude evolved from extension dominated transtension prior to 8 ma to dextral shear dominated transtension by 7 6 ma the onset of dextral shear in coastal sonora likely resulted from an increase in rift obliquity due to a change in relative plate motion direction at 8 ma the increase in rift obliquity and resultant onset of significant strike slip faulting played a crucial role in facilitating subsequent plate boundary localization and marine incursion in the northern gulf of california by 6 ma and continental rupture at 2 1 ma

the stress heat flow paradox and thermal results from cajon pass conventional friction models predict a substantial thermal anomaly associated with active traces of strike slip faults but no such anomaly is observed from over 100 heat flow determinations along 1 000 km of the san andreas fault the cajon pass well is being drilled to bring deep heat flow and stress data to bear on this paradox preliminary heat flows to 1 7 km in the neighboring arkoma well indicated a positive anomaly of about 30 as would be expected if frictional resistance of the fault were 50 mpa or so but the pliocene and pleistocene history of the transverse ranges particularly high rates of uplift and erosion could also explain the anomaly preliminary stress results from cajon pass and a new interpretation of regional data by mark d zoback and colleagues suggests that the maximum compressive stress near the fault is almost normal to the trace and hence the resolved shear stress is low and the fault weak although this is consistent with long standing evidence from low heat flow vexing questions remain regarding the physics of strike slip faulting the heat flow data show large variability with depth probably from three dimensional structure and an overall decrease from over 90 mw m in the upper kilometer to less than 80 mw m in the lower 300 m with no evidence of advective heat transfer whether this decreasing trend is significant and whether it continues downward to the background value 70 mw m less the radiogenic heat from overlying rocks as expected for a weak fault must be verified with data from greater depth

production of 3he in crustal rocks by cosmogenic thermal neutrons accurate determination of cosmogenic 3he concentrations in minerals has to account for nucleogenic 3he produced from the reaction of thermalized neutrons with 6li thermal neutrons from the cosmic ray cascade are typically not considered however in exposed basalts and granites the flux of cosmogenic thermal neutrons ctn is generally 10 200 times higher than the thermal neutron flux produced by n reactions in the rock with the absolute values dependent on composition and geographic location the reaction of ctn with li can account for 5 50 of the cosmogenic 3he in minerals in typical granitiod rocks depending on the li content and distribution and petrography of the rock and may be considerably higher in special cases in basalts this contribution is usually lower 1 5 6 but may still warrant consideration the strong depth dependence of ctn produced 3he in rock may be used to assess erosion of landforms similar to ctn produced 36cl however as 3he is stable it has the potential to extend the useful time range to older surfaces

quantifying the energy dissipation of overriding plate deformation in three dimensional subduction models in a subduction system the force and the energy required to deform the overriding plate are generally thought to come from the negative buoyancy of the subducted slab and its potential energy respectively such deformation might involve extension and back arc basin formation or shortening and mountain building how much of the slab s potential energy is consumed during overriding plate deformation remains unknown in this work we present dynamic three dimensional laboratory experiments of progressive subduction with an overriding plate to quantify the force fopd that drives overriding plate deformation and the associated energy dissipation rate opd and we compare them with the negative buoyancy fbu of the subducted slab and its total potential energy release rate bu respectively we varied the viscosity ratio between the plates and the sublithospheric upper mantle with sp um 157 560 and the thickness of the overriding plate with top 0 5 2 5 cm scaling to 25 125 km in nature the results show that fopd fbu has average values of 0 5 2 0 with a maximum of 5 3 and opd bu has average values of 0 05 0 30 with a maximum of 0 41 the results indicate that only a small portion of the negative buoyancy of the slab and its potential energy are used to deform the overriding plate our models also suggest that the force required to deform the overriding plate is of comparable magnitude as the ridge push force furthermore we show that in subduction models with an overriding plate bending dissipation at the subduction zone hinge remains low 3 15 during steady state subduction

submarine flow tills at victoria british columbia sea cliffs at victoria expose submarine flow tills within an ice contact complex of flow till outwash proglacial cones interbedded with dinoflagellate bearing glaciomarine fine sediments glacial debris flow lobes mainly flow tills were formed by supraglacially englacially and basally transported sediment slumping off an ice margin grounded in the strait of juan de fuca the structures and textures reveal that lobes formed by a conveyor mechanism in some lobes a double sorting phenomenon apparently developed involving an outer winnowed layer of pseudo plastic flow around an inner core of till that was sorted by dewatering and expulsion of fines outward from the centre during flow structures and fabric data indicate lobe travel was gravitationally controlled down cone flanks and independent of ice movement a depositional model for the complex proposes the ice margin produced a proglacial apron of interdigitating cones which accumulated on the sea floor sediment facies variations were probably c

modeling local crustal isostasy caused by ductile flow in the lower crust some continental regions display local isostatic uplift on a much shorter scale than that of classical isostasy such as high ground associated with granites this is best explained by compensatory channel flow in the lower crust to investigate the mechanism the uplift caused by an existing granitic body 30 km wide extending to 10 km depth and of density deficiency 120 kg m 3 has been modeled by finite element analysis as it rises toward isostatic equilibrium over 5 myr by inward creep in the ductile lower crust the upper crust is treated as elastoplastic with a specified strength envelope the lower crust 15 to 36 km depth is viscoelastic with either newtonian or power law quartz rheology and the strong upper mantle down to 60 km is either elastoplastic or has olivine rheology models with channel flow produce much higher and sharper topography than those without it faulted models produce the highest and sharpest elevations when preexisting faults are present the uplift is strongly affected by the state of stress in the upper crust with compression suppressing uplift on outward dipping faults and tension accentuating it the opposite for inward dipping faults the surface topography approaches isostatic equilibrium over 5 myr at a decaying rate dependent on the type of rheology the moho is pulled upward as the surface rises during the earlier stages but it eventually subsides toward its equilibrium level much more slowly than the surface approaches equilibrium this linked deformation of surface and moho allows approximate isostatic equilibrium of the upper crust to be established without inflow at the edges of the model and it may produce long lasting bumps on the moho

the paradox of the axial profile isostatic compensation along the axis of the mid atlantic ridge mantle bouguer gravity anomalies mba and bathymetry on three profiles covering more than 1000 km along the axis of the mid atlantic ridge mar are highly correlated suggesting that along axis topographic relief is locally compensated by variations in crustal thickness and or mantle density structure the quantitative relationship between topography and gravity on these profiles could be explained by the flexure of a thin narrow elastic strip representing the response to isostatic loads of an inner rift valley isolated from the rest of the plate by weak bounding normal faults the paradox is that across axis profiles show that the median valley is an uncompensated feature apparently created by a dynamic mechanism new extensive off axis coverage of the mar from 31 to 36 s shows that the high correlation does not persist outside the axial zone we suggest that the on axis correlation exists because the mechanism creating the median valley is controlled by the mantle thermal structure and along axis variations in crustal thickness both of which contribute to the mba if the mechanism is extension of a brittle ductile lithosphere the critical parameter controlling topographic relief is the thickness of the relatively stiff mantle layer immediately beneath the crust at the ridge axis a three dimensional 3 d thermal model incorporating passive mantle flow hydrothermal circulation the plate boundary geometry and variable magmatic heating associated with observed variations in crustal thickness predicts variations in the thickness of the stiff mantle layer that correlate with the observed axial topography we model the expected topography using cross axis sections of the 3 d thermal model and a 2 d finite element model of an extending lithosphere that incorporates temperature and strain dependent rheology as well as the flexural response of a thickening plate the predicted topographic signal produced by the combined dynamic and isostatic effects matches the amplitude of the observed axial bathymetry at the 18 mm yr 1 spreading half rate in our south atlantic survey the 2 d models also reproduce the observed rapid transition along the axis between rift valley no rift valley morphology we conclude that extensional forces acting on segmented oceanic lithosphere with varying rates of crustal production produce the highly variable morphology of the southern mar and the along axis correlation between mba and bathymetry

tectonic analysis of the dead sea rift region since the late cretaceous based on mesostructures we have measured the orientations of small faults slickensides tectonic stylolites vein with secondary mineralization small folds and dikes in israel and sinai these structures are indicators of paleo stress or strain and provide the pattern of tectonic deformation data were collected at 130 stations most with tens of separate measurements most stations showed consistent deformation stations were located on exposures ranging from precambrian crystalline rocks to pleistocene sediments we have defined two tectonic stress fields each relatively uniform in both time and space one stress field with dominating maximum horizontal compression trending w to wnw in the late cretaceous to eocene rocks in the folds and plateaus west of the dead sea rift the second field with dominating horizontal extension trending e to ene in all rocks inside the rift and proximal thereto the first stress field is called the syrian arc stress and the second is called the dead sea stress a change in style of tectonic deformation which corresponds to the two stress fields is manifested also in the major structures in israel the late cretaceous to neogene deformation is characterized by long wavelength folds and monoclines whereas the neogene to recent deformation is characterized by normal and strike slip faults and volcanic activity

stress orientation profile to 3 5 km depth near the san andreas fault at cajon pass california a detailed profile of the orientation of the apparent maximum horizontal compressive stress was constructed over the depth interval 1 75 3 46 km in the cajon pass drill hole located 4 2 km from the san andreas fault in southern california the profile is based on the analysis of stress induced well bore breakouts and consists of 32 616 orientation determinations at a basic sampling interval of 4 cm the azimuth of the apparent maximum horizontal compressive stress around the borehole is 057 s d 19 depth dependent variations in breakout azimuths from a few degrees to as much as 100 occur over depth intervals of several centimeters to hundreds of meters in the borehole with wavelengths showing a self similar distribution over a range of depth intervals the variations in breakout orientation at depth seem to reflect stress fluctuations associated with active faults penetrated by the drill hole assuming linear elastic behavior perturbations in stress magnitude and orientation were computed for predrilling slip on these faults fault stress drops limited to the ambient shear stress i e up to total stress drop along with occasional local extreme stress drops and geometrical complications can explain the variations in breakout orientations in the drill hole the penetrative stress inhomogeneity may suggest that the average orientation of borehole breakouts in cajon pass which indicates left lateral shear on planes parallel to the san andreas fault may not be representative of the state of stress near the fault at depth in this region rather the stress orientation profile indicates the superposition of numerous local stress perturbations on an average stress field which is characterized by an absence of appreciable right lateral shear on planes parallel to the san andreas combining these observations with the highly variable stress state in shallow boreholes in the western mojave desert it is suggested that the heterogeneous stress field in this region representing a spectrum of seismic events may extend over substantial distances across and along the san andreas fault zone if the average measured breakout orientation in cajon pass is representative of the state of stress near the san andreas in this region then substantial left lateral shear stress is resolved on planes parallel to the fault in contradiction to the sense of its long term motion we use an elastic dislocation model to compute the net stress change near the cajon pass drill site since just prior to the large 1812 earthquake taking into account coseismic slip in major earthquakes and aseismic accumulation of slip on the san andreas and san jacinto faults the model suggests that the net change in fault parallel shear stress during the current earthquake cycle in the cajon pass area is nearly negligible the difference between the measured and computed results requires that additional left lateral shear stress be superimposed on a generally fault normal maximum horizontal compressive stress the results suggest that during the great fort tejon earthquake of 1857 left lateral shear stress parallel to the san andreas fault in the region of cajon pass may have played a role in terminating the southeastward extent of the rupture propagation

determination of 36 cl production rates derived from the well dated deglaciation surfaces of whidbey and fidalgo islands washington the 36 cl dating method is increasingly being used to determine the surface exposure history of quaternary landforms production rates for the 36 cl isotopic system a critical component of the dating method have now been refined using the well constrained radiocarbon based deglaciation history of whidbey and fidalgo islands washington the calculated total production rates due to calcium and potassium are 91 5 atoms 36 cl g ca i1 yr i1 and are 228 18 atoms 36 cl g k i1 yr i1 respectively the calculated ground level secondary neutron production rate in air pf 0 inferred from thermal neutron absorption by 35 cl is 762 28 neutrons g air i1 yr i1 for samples with low water content 1 2 wt neutron absorption by serpentinized harzburgite samples of the same exposure age having higher water content 8 12 wt is 40 greater relative to that for dry samples these data suggest that existing models do not adequately describe thermalization and capture of neutrons for hydrous rock samples calculated 36 cl ages of samples collected from the surfaces of a well dated dacite flow 10 600 12 800 cal yr b p and three disparate deglaciated localities are consistent with close limiting calibrated 14 c ages thereby supporting the validity of our 36 cl production rates integrated over the last 15 500 cal yr between latitudes of 46 5 and 51 n although our production rates are internally consistent and yield reasonable exposure ages for other localities there nevertheless are significant differences between these production rates and those of other investigators c 2001 university of washington

patterns of stress at midocean ridges and their offsets due to seafloor subsidence the effect of the seafloor subsidence on the horizontal stress field is investigated by combining the finite element method with a formulation that allows us to compute the two dimensional 2d horizontal stresses arising from isostatically compensated vertical loads the topographic load created by the elevation of midocean ridges relative to old ocean floor is shown to be a significant source of ridge parallel tensile stresses these may predominate over the ridge perpendicular stresses and explain observations at midocean ridge offsets such as 1 oblique normal faulting at ridge transform intersections trending up to 60 relative to the ridge axis and 2 nontransform offsets consisting of structures oriented at 45 relative to the ridge trend at midocean ridge overlaps rotation of the ridge parallel tensile stresses favours rift propagation at more than 45 relative to the ridge trend it is suggested that propagating rift tips that bend abruptly lead to partially unlocked offsets and as a result large overlaps may eventually start to rotate and evolve into a microplate

53mn profiles in four apollo surface cores 53mn data are presented for four apollo cores with an emphasis on the past 107 year history of these samples 1 core 15011 has excess 53mn down to a depth of at least 9 cm which is probably due to the continuous addition of near surface irradiated material high 53mn was also observed at 17 cm which suggests that this might have been the surface at this site 7 10 m y ago the soil found covering the 15011 core tube keeper appears to be surface soil but most likely not from the top of 15011 2 the 53mn profile for 64002 above 12 cm is relatively flat and in excess of that calculated for a static core which suggests that the upper part of this core was deposited rapidly about 2 m y ago as this soil appears to be mature well irradiated local material it seems unlikely that it is ejecta from south ray crater unless south ray crater is much older than 2 m y 3 results for the apollo 17 drill stem show a large excess of 53mn from at least 3 cm downwards some magnetic separates obtained from this core have very high 53mn activities probably due to the presence of agglutinates that were recently irradiated very near the surface these results are consistent with the suggestion of the battelle northwest group that this core sampled a young crater that had partially filled due to the slumping of surface irradiated material 4 the 53mn content of the upper part of 74002 is similar to that of trench samples collected nearby and indicates that the gray orange and black soils at this site have been exposed together near the surface for at least the last 10 m y

modelling the evolution of asymmetrical basins bounded by high angle reverse faults with application to foreland basins abstract the evolution of asymmetrical basins bounded by high angle reverse faults in a 10 and 20 km thick strong upper crust with a byerlee type strength envelope has been modelled by finite element analysis the substratum is assumed to be inviscid and the basin is filled to datum by sediments the results are compared with those of comparable extensional half grabens as previously studied as the layer is shortened by progressive increments zones of plastic failure extend until throughgoing failure occurs with the basin progressively narrowing as it deepens at this stage further subsidence becomes negligible placing a limit on the depth which depends on the layer thickness and sediment density during the earlier stages of evolution the flexure profiles are very similar to those of the otherwise equivalent extensional half graben but during later stages substantially deeper and narrower compressional basins can form because of the greater crustal strength under compression beyond throughgoing failure further shortening causes plastic compressional folding which develops on top of the earlier gentle flexural undulations eventually producing quite strong anticlines and synclines of progressively decreasing wavelength which decay away from the fault on both sides compressional basins as modelled range from under 25 km to over 150 km in width and they can be up to nearly 8 km deep with greater widths and depths for a thicker layer the most important application of the results is to the origin of foreland basins and foredeeps providing an alternative and possibly complementary mechanism to the supracrustal loading hypothesis for the formation of such basins in particular the asymmetrical thrust basin hypothesis explains how the narrow and deep foreland basins characteristic of european mountain belts can form

modeling the formation of a half graben using realistic upper crustal rheology the evolution of a half graben in a strong upper crust with a byerlee strength envelope underlain by an inviscid substratum has been modeled using elasto viscoplastic finite element analysis the nonlithostatic stresses regions of failure and flexure profiles have been computed as the layer is extended by increments the weaker upper part of the layer fails on stretching and more extensive regions of tensional failure extend progressively downward beneath the downbending half graben and upward from the bottom of the layer beneath the footwall uplift compressional failure occurs locally due to bending or unbending plastic yielding progressively reduces the widths of the half graben and footwall uplift as extension proceeds as the outer hinge line of the half graben migrates toward the fault the plastic deformation reduces the effective elastic thickness of the layer by up to 67 helping to explain the anomalously low observed values associated with normal faulting throughgoing failure in tension eventually occurs beneath the footwall uplift near the fault when subsidence slows down and ceases the strength of the upper crust thus limits the amount of subsidence that can occur for an 8 km thick layer and sediment infill 400 kg m3 less dense than the basement the half graben width decreases from over 80 km at the onset of extension to 42 km when the subsidence has reached its limit of 2 3 km the equivalent elastic layer thickness is then reduced to 3 3 km thicker layers have correspondingly greater widths and limiting depths less dense sediments result in a narrower half graben and smaller limiting depth

localization and fault growth in layered brittle ductile systems implications for deformations of the continental lithosphere we present experiments and numerical simulations dealing with the growth of faults in thin brittle ductile systems to understand deformation modes in the continental lithosphere experiments were uniaxial shortening of layers of dry sand and silicone putties of various viscous resistances for large strength ratios between the brittle and ductile layers r 5 10 the deformation localizes into two shear bands the fault pattern is created before reaching 10 shortening and has fractal dimensions varying between 1 6 and 1 8 for small strength ratios r 5 10 deformation never localizes the fault pattern is homogeneous with a trivial dimension of 2 and grows continuously during deformation the transition between localized and homogeneous deformation occurs when the mechanical resistance of brittle layers is 5 10 times larger than the resistance of ductile layers this transition was also investigated by means of electrical analog simulations a fuse network which represents an elasto brittle layer is coupled with a capacitor layer which models strain rate dependent fluids an ac potential is applied and the fuses progressively burned out until they form a connected network the ac potential frequency is a tuning parameter similar to the applied strain rate in experiments a critical frequency is obtained marking a transition between a localization mode where the density of burned fuses decreases as the system size increases and a delocalization mode where the density of burned fuses remains constant with increasing system size the scaling dependency of the fracture process as well as the critical frequency are consistent with experimental results available information on the rheology of the continental lithosphere shows that this mechanical transition is bracketed by the possible range of brittle to ductile strength ratios

geological evidence for a global tectonic polarity inferences about plate motions over the past 40 ma based on structural and geodetic data suggest that there is a coherent movement direction on a global scale moreover a westward drift of the lithosphere relative to the asthenosphere is indicated by plate motions in the hot spot reference frame independent geological observations such as the different styles of deformation in thrust belts associated with subduction following or opposing the relative eastward or northeastward mantle flow also support this global tectonic polarity unfortunately most hot spot reference frames are based on assumptions which strongly limit their validity for example many combine magmatic sources originating from different depths in the mantle including hot spots located along oceanic ridges which are not fixed relative to one another using a hot spot reference frame filtered to exclude shallow hot spots and anomalous wet spots we would predict a much higher relative westward drift of the lithosphere plate motions are not random rather that they are controlled by the relative motion between lithosphere and asthenosphere and by gradients in lithospheric thickness and composition triangular shaped plates may experience rotations resulting from differential drag at the base of the lithosphere with respect to the asthenosphere e g the clockwise rotation of south america plates appear to follow an undulatory pattern of motion on a global scale and their poles of rotation fall into a main cone located near the earth s rotation poles plate rotations induced by triangular shape differential basal drag or lithospheric anisotropies along subduction zones may generate the scattered distribution of euler poles this might have inhibited the recognition of a coherent global tectonic pattern

modern tectonic stress field in the mediterranean region evidence for variation in stress directions at different scales summary a compilation of more than one thousand stress indicators which include in situ stress measurements focal mechanisms microtectonic and other geological data allowed us to reconstruct the modern stress field in the mediterranean region and the surrounding area average stress directions at different scales have been reconstructed by means of a linear interpolation method this method takes into account the distribution scale and quality of stress data the results of the interpolation at plate scale allow us to recognize slightly deformed regions such as the northwestern european platform where average maximum horizontal stress direction is oriented roughly nnw sse subparallel to absolute and relative plate velocity directions other regions such as the caucasus alps and pyrenees where recent tectonic deformation and seismicity are present display important variations of stress directions the reconstruction of the average stress directions at different scales within the french alps pointed out that the average stress field pattern may vary from one scale to another nevertheless variations of stress directions at a given scale are consistent with the kinematics of faults of the same scale

the influence of pre existing thrust faults on normal fault geometry in nature and in experiments relations between normal faults and pre existing thrust faults are classically described in terms of three basic situations normal faults can cross cut thrust faults they can branch out from thrust faults at depth on a de collement level or they can entirely reactivate thrust planes the mechanical aspects of these types of interaction were studied by analogue modelling in which sand simulates the brittle rocks and silicone putty an interlayered de collement the models underwent compression producing thrust faults with variable dips followed by extension three possible ways of interaction are described here a no interaction occurs in the case of low dip thrust faults 32 1 and normal faults are developed independently displaying a listric geometry b branching at depth on the de collement level occurs when dip of the thrust faults reaches 32 1 in this case the dip of the normal faults whose geometry becomes planar decreases with increasing thrust dip we suggest that this change in dip of normal faults depends upon the rotation of stress tensor axes along the pre existing fault zone where a drop in the friction coefficient is likely to occur c reactivation occurs in brittle material when dip of the pre existing fault exceeds 41 1

deformation and stress states along the central segmentof the philippine fault implications to wrench fault tectonics abstract this paper discusses the space time evolution of deformation and states of stress along the central segment of the left lateral philippine fault from bondoc peninsula to leyte island this segment is characterized by deformations and stress states which do not always conform to predictions of classical wrench tectonics directions of the maximum principal stress axis 1 are in some cases oriented perpendicular to the fault trace contradicting the classical fault strike 1 angle of 30 45 in other cases it is the minimum axis 3 that orients perpendicular to the fault or in other cases parallel to it rotations of 1 directions as functions of time and space are observed all along the fault more complex stress regimes are observed in areas where the philippine fault intersects with two other major left lateral strike slip faults namely the legaspi lineament in the ragay gulf area and the sibuyan sea fault in masbate these observations strongly underline the urgence of prudence and precaution in applying wrench fault tectonic principles in natural examples

synopsis sfb 108 stress and stress release in the lithosphere1 abstract stress and stress release in the lithosphere has been studied within a collaborative research centre at karlsruhe university sfb 108 as special contribution from germany to the international lithosphere program ilp this synopsis is an outline of the research results obtained by the sfb 108 as presented in this special volume of tectonophysics it starts with a summary of ideas on stress and stress release in the lithosphere which guided the sfb 108 at its beginning in 1981 it is followed by a review of strategy in cooperative research on rift systems world stress map and the scales of structure of the continental lithosphere from crust to mantle the main observations are then summarized and connections are made between key results from the various disciplines to obtain new views in lithospheric tectonics finally we address open questions and perspectives for future research the research addressed various styles of continental rifting from the afro arabian rift system to the central european rift system with the rhinegraben and the grabens in the french massif central as outstanding features a number of critical parameters such as thickness heat flow rheology composition strength of the crust and of the lithospheric mantle and the influence of active mantle plumes were identified and classified together they strongly influence the style of rifting petrophysical modelling of integrated information on seismic velocities composition and temperature from xenolith analysis density gravity and topography provided essential constraints on plume lithosphere interaction in the french massif central and also in the kenya rift the rhinegraben and its variscan environment offered the opportunity to study different concepts of upper and lower crust including the occurrence of deep crustal earthquakes and the role of an alpine detachment a special concern of the sfb 108 was the distribution of dimensions of heterogeneities in the lithosphere and its relation to the scales of their generating processes the crust mantle boundary was recognized as a major change in scales of heterogeneities by deep seismic sounding dss methods applied to the lower crust and the lithospheric mantle the study of the tectonic stress in western europe revealed that the crust is in frictional equilibrium down to midcrustal levels and is also decoupled from the mantle allowing adjustive motions of blocks and independent subduction in the foreland of the alps this decoupling is achieved in a thin and hot lithosphere driven by mid atlantic ridge push and alpine continent continent collision it is described in a quantitative dynamic model of crust mantle decoupling the sfb 108 has shown that earth scientists have to rediscover the importance of pattern recognition if complex influences are to be recognized

impact of cenozoic strike slip tectonics on the evolution of the northern levant basin offshore lebanon sedimentary basins adjacent to plate boundaries contain key tectonic and stratigraphic elements to understand how stress is transmitted through plates the levant basin is a place of choice to study such elements because it flanks the levant fracture system and the africa anatolia boundary this paper uses new high quality 3 d seismic reflection data to unravel the tectonic evolution of the margin of this basin during the cenozoic the period corresponding to the formation of the levant fracture system part of the africa arabia plate boundary four major groups of structures are identified in the interpreted cenozoic units nw se striking normal faults nne ssw striking thrust faults ene wsw striking dextral strike slip faults and nne trending anticlines we demonstrate that all structures apart of the nw se striking normal faults are inherited from mesozoic faults their reactivation and associated folding started during the late miocene prior to the messinian salinity crisis due to a nw se compressional stress field no clear evidence of shortening at present day offshore lebanon and no large nne ssw strike slip faults parallel to the restraining bend are found indicating that the levant fracture system is mainly contained onshore at present day the intermittent activity of the interpreted structures correlates with the two stages of levant fracture system movement during late miocene and pliocene this paper provides a good example of the impact of the evolution of plate boundaries on adjacent basins and indicates that any changes in the stress field as controlled by the plate boundary will affect immediately the preexisting structures in adjacent basins

structure and mechanics of the hayward rodgers creek fault step over san francisco bay california a dilatational step over between the right lateral hayward and rodgers creek faults lies beneath san pablo bay in the san francisco bay area a key seismic hazard issue is whether an earthquake on one of the faults could rupture through the step over enhancing its maximum possible magnitude if ruptures are terminated at the step over then another important issue is how strain transfers through the step we developed a combined seismic reflection and refraction cross section across south san pablo bay and found that the hayward and rodgers creek faults converge to within 4 km of one another near the surface about 2 km closer than previously thought interpretation of potential field data from san pablo bay indicated a low likelihood of strike slip transfer faults connecting the hayward and rodgers creek faults numerical simulations suggest that it is possible for a rupture to jump across a 4 km fault gap although special stressing conditions are probably required e g harris and day 1993 1999 slip on the hayward and rodgers creek faults is building an extensional pull apart basin that could contain hazardous normal faults we investigated strain in the pull apart using a finite element model and calculated a 0 02 mpa yr differential stressing rate in the step over on a least principal stress orientation nearly parallel to the strike slip faults where they overlap a 1 to 10 mpa stress drop extensional earthquake is expected on normal faults oriented per pendicular to the strike slip faults every 50 500 years the last such earthquake might have been the 1898 m 6 0 6 5 shock in san pablo bay that apparently pro duced a small tsunami historical hydrographic surveys gathered before and after 1898 indicate abnormal subsidence of the bay floor within the step over possibly related to the earthquake we used a hydrodynamic model to show that a dip slip mechanism in north san pablo bay is the most likely 1898 rupture scenario to have caused the tsunami while we find no strike slip transfer fault between the hayward and rodgers creek faults a normal fault link could enable through going segmented rupture of both strike slip faults and may pose an independent hazard of m 6 earthquakes like the 1898 event

hydraulic fracturing stress measurements at yucca mountain nevada and relationship to the regional stress field hydraulic fracturing stress measurements and acoustic borehole televiewer logs were run in holes usw g 1 and usw g 2 at yucca mountain as part of the nevada nuclear waste storage investigations for the u s department of energy eight tests in the saturated zone at depths from 646 to 1288 m yielded values of the least horizontal stress s h that are considerably lower than the vertical principal stress s v in tests for which the greatest horizontal principal stress s h could be determined it was found to be less than s v indicating a normal faulting stress regime the borehole televiewer logs showed the presence of long in excess of 10 m vertical drilling induced fractures in the first 300 m below the water table these are believed to form by the propagation of small preexisting cracks under the excess downhole fluid pressures up to 5 2 mpa applied during drilling the presence of these drilling induced hydrofractures provides further confirmation of the low value of the least horizontal stresses a least horizontal principal stress direction of n60 w n65 w is indicated by the orientation of the drilling induced hydrofractures n25 e n30 e and the orientation of stress induced well bore breakouts in the lower part of usw g 2 n65 w this direction is in good agreement with indicators of stress direction from elsewhere at the nevada test site the observed stress magnitudes and directions were examined for the possibility of slip on preexisting faults using these data the coulomb criterion for frictional sliding suggests that for coefficients of friction close to 0 6 movement on favorably oriented faults could be expected for coefficients of friction of 1 0 preexisting faults of all orientations should be stable laboratory studies on the yucca mountain tuffs reported elsewhere yield coefficients of friction ranging from 0 6 to 0 9

status of the crustal stress in egypt as inferred from earthquake focal mechanisms and borehole breakouts abstract the recent stress field has been investigated by the analysis of the directions of maximum horizontal stress h as derived from earthquake focal mechanisms and borehole breakouts in egypt the results indicate that in the southern part of the gulf of suez the shallow borehole breakouts stress directions are not consistent with the deep earthquakes focal mechanisms stress directions analysis of 30 borehole breakouts measurements indicate wnw ese alignment of the maximum horizontal stress with a vector mean of 110 14 p e however the earthquake studies including orientation of p axes reflect ene wsw alignment of h with a vector mean of 66 30 thus the stress directions are not constant throughout the crust along the gulf of aqaba the southern part of the dead sea fault the focal mechanism solutions indicate that the maximum horizontal stress is presently nw se oriented corresponding to strike slip mechanism in concert with the geological evidence in northern egypt the nine earthquake focal mechanisms for the nile delta cairo bitter lakes region scatter in orientation varying from ne sw to nw se for h about 45 from these mechanisms indicate nw se 22 wnw ese 22 ene wsw and 11 ne sw alignment of the maximum horizontal stress while in southern egypt it is relatively uniform and aligned to nearly e w direction with a vector mean of 95 3

finite element modelling of stress patterns along the mid norwegian continental margin 62 to 68 n the inferred nature of the regional intraplate stress field off the coast of norway is based primarily on borehole breakout information and earthquake focal mechanisms the stress indicators reveal a slightly clockwise rotation of the orientation of the principal horizontal stress sh to the northwest with respect to the dominant e w orientation observed in the northern north sea the stress regime is predominantly strike slip and thrust fault although two normal faulting events have occurred on the shelf and shelf edge within the mid norwegian margin segment 62 to 68 n the tr0ndelag platform exhibits large variations of observed sn orientations this may be due to an almost isotropic horizontal stress state sh sh resulting from the superposition of local and regional stress sources this paper presents the predicted tectonic stresses along the mid norwegian margin based on a 3 d finite element analysis the two principal modelling objectives are to elaborate the effects of 1 changes in the regional stress magnitude sh and 2 variations in rock properties i e young s modulus on the horizontal stress anisotropy hsa of the v0ring basin and the tr0ndelag platform the model incorporates simplified mainland topography the tr0ndelag platform the voring basin the v0ring plateau and the lofoten basin our modelling results indicate that horizontal stress anisotropy hsa and the regional sh magnitude within the various structural units is dependent on their strike with respect to the orientation of the present day regional stress field specifically we find 1 a near isotropic horizontal stress magnitude configuration for the southern part of the v0ring basin where the gradient of topography across the mid norwegian margin is oriented oblique to the regional trend of sh 2 a maximum horizontal stress anisotropy hsa in the southern part of the tr0ndelag platform this local hsa maximum bordering the hsa minimum of the voting basin 3 a narrow band of high hsa at the eastern edge of the tr0ndelag platform imposed by compressive gravitational stresses related to the elevated topography of mainland norway 4 a low hsa at the western margin of the tr0ndelag platform related to spreading stresses associated with the topographic lowering towards the v0ring basin and 5 in case of a far field stress of sh 1 1sh the modelling results predict exchanged principal horizontal stresses sh and sh at the western margin of the tr0ndelag platform

ductile creep compaction and rate and state dependent friction within major fault zones the shear traction on major strike slip faults during earthquakes is much lower than that expected on a frictionally sliding surface in equilibrium with hydrostatic pressure the low shear traction is explained if the fluid pressure at the time of the earthquake is much greater than hydrostatic pressure ductile creep within mostly sealed fault zones compacts the matrix and thus increases fluid pressure between earthquakes frictional dilatancy during earthquakes decreases fluid pressure below hydrostatic and over the earthquake cycle the fault zone is in long term equilibrium with the country rock this ductile mechanism is formally unified with rate and state theory for time dependent friction when the difference between a critical porosity where the rock loses all strength and the actual porosity of cracks is used as a state variable this choice is justified by percolation theory of mostly broken lattices time dependent behavior associated with changes in normal traction in the laboratory is explained by the formalism instability earthquakes sometimes occurs in the numerical experiments however fairly small amounts of frictional dilatancy during initial frictional creep decrease fluid pressure and preclude unstable sliding two coupled mechanisms for producing dilatancy on faults once an instability is well underway are evident 1 expansion of pore fluids associated with frictional heating increases fluid pressure offsetting the effects of increased pore volume during earthquakes there is some tendency for pore volume increase to balance fluid expansion so that fluid pressure stays relatively constant 2 production of isolated voids that do not immediately decrease fluid pressure throughout the fault zone during earthquakes can occur to the extent that the fault zone is not significantly strengthened although the extent of both processes is constrained by energy considerations the variation of fluid pressure during earthquakes is not yet well enough understood to predict stress drop from observable material properties

landscape evolution of the dry valleys transantarctic mountains tectonic implications there are different views about the amount and timing of surface uplift in the transantarctic mountains and the geophysical mechanisms involved our new interpretation of the landscape evolution and tectonic history of the dry valleys area of the transantarctic mountains is based on geomorphic mapping of an area of 10 000 km2 the landforms are dated mainly by their association with volcanic ashes and glaciomarine deposits and this permits a reconstruction of the stages and timing of landscape evolution following a lowering of base level about 55 m y ago there was a phase of rapid denudation associated with planation and escarpment retreat probably under semiarid conditions eventually downcutting by rivers aided in places by glaciers graded valleys to near present sea level the main valleys were flooded by the sea in the miocene during a phase of subsidence before experiencing a final stage of modest upwarping near the coast there has been remarkably little landform change under the stable cold polar conditions of the last 15 m y it is difficult to explain the sirius group deposits which occur at high elevations in the area if they are pliocene in age overall denudation may have removed a wedge of rock with a thickness of over 4 km at the coast declining to 1 km at a point 75 km inland which is in good agreement with the results of existing apatite fission track analyses it is suggested that denudation reflects the differences in base level caused by high elevation at the time of extension due to underplating and the subsequent role of thermal uplift and flexural isostasy most crustal uplift 2 4 km is inferred to have occurred in the early cenozoic with 400 m of subsidence in the miocene followed by 300 m of uplift in the pliocene

sources of recent tectonic stress in the pannonian region inferences from finite element modelling summary we present the results of finite element modelling of the recent stress field in the pannonian basin and surrounding alpine orogenic belt our results show that the recent predominantly compressive stress regime in the alpine pannonian carpathian dinaric system is governed by distinct tectonic factors of great importance is the deformation of crustal blocks with diverent geometries and rigidities in an overall convergent setting associated with the africa europe collision the most important stress source appears to be the counterclockwise rotation of the adriatic microplate at the southwest boundary of the pannonian basin this plate tectonic unit has been interpreted as moving independently of both the european plate and the african plate additional boundary conditions active shortening and compression in the vrancea zone and the bohemian massif and the evect of the immobile moesian platform also significantly influence the modelling results the incorporation of additional stress sources such as crustal thickness variation and the presence of two main fault zones separating the primary tectonic units in the study area have only locally important evects but improve the fit between the calculated results and the observed stress pattern

continental tectonics in the aftermath of plate tectonics the success of plate tectonics required an acceptance of continental drift and thus a reinterpretation of the large scale geological history of most of the earth but the basic tenet of plate tectonics rigid body movements of large plates of lithosphere fails to apply to continental interiors where buoyant continental crust can detach from the underlying mantle to form mountain ranges and broad zones of diffuse tectonic activity

the production rate of cosmogenic 38ar from calcium in terrestrial pyroxene abstract pyroxene separated from dolerite surfaces which were exposed to cosmic ray irradiation for millions of years in the antarctic dry valleys contains considerable amounts of cosmogenic 38ar as evident from 38ar 36ar ratios up to 0 2887 air 0 1880 in five out of eight mineral separates the concentrations of k ti and cl are low enough to enable us to deduce the terrestrial production rate of 38ar from ca the remaining relevant target element by relating cosmogenic 38ar concentrations to 3he and 21ne exposure ages correlations of cosmogenic 38ar normalized to ca to total 3he and to cosmogenic 21ne normalized to mg respectively are excellent however various currently used methods to derive the 3he and 21ne production rates in pyroxene lack consistency depending on the method chosen the 38ar production rate derived here ranges between 191 21 atoms g ca 1 a 1 and 254 28 atoms g ca 1 a 1 at sea level standard atmospheric pressure and high latitude a more accurate value may be calculated from our data as soon as the he and ne production rates from pyroxene are known more reliably our production rate determination is largely independent of past geomagnetic field variations and of the scaling method used

cosmic ray exposure and pre atmospheric size of the gebel kamil iron meteorite cosmogenic he ne and ar as well as the radionuclides 10be 26al 36cl 41ca 53mn and 60fe have been determined on samples from the gebel kamil ungrouped ni rich iron meteorite by noble gas mass spectrometry and accelerator mass spectrometry ams respectively the meteorite is associated with the kamil crater in southern egypt which is about 45 m in diameter samples originate from an individual large fragment individual as well as from shrapnel concentrations of all cosmogenic nuclides stable and radioactive are lower by a factor 3 4 in the shrapnel samples than in the individual assuming negligible 36cl decay during terrestrial residence indicated by the young crater age 85 cm i e a pre atmospheric mass 20 tons with a preferred radius of 115 120 cm 50 60 tons the analyzed samples came from a depth of approximately 20 cm individual and approximately 50 80 cm shrapnel the size of the gebel kamil meteoroid determined in this work is close to estimates based on impact cratering models combined with expectations for ablation during passage through the atmosphere folco et al 2010 2011

ductile creep and compaction a mechanism for transiently increasing fluid pressure in mostly sealed fault zones a simple cyclic process is proposed to explain why major strike slip fault zones including the san andreas are weak field and laboratory studies suggest that the fluid within fault zones is often mostly sealed from that in the surrounding country rock ductile creep driven by the difference between fluid pressure and lithostatic pressure within a fault zone leads to compaction that increases fluid pressure the increased fluid pressure allows frictional failure in earthquakes at shear tractions far below those required when fluid pressure is hydrostatic the frictional slip associated with earthquakes creates porosity in the fault zone the cycle adjusts so that no net porosity is created if the fault zone remains constant width the fluid pressure within the fault zone reaches long term dynamic equilibrium with the hydrostatic pressure in the country rock one dimensional models of this process lead to repeatable and predictable earthquake cycles however even modest complexity such as two parallel fault splays with different pressure histories will lead to complicated earthquake cycles two dimensional calculations allowed computation of stress and fluid pressure as a function of depth but had complicated behavior with the unacceptable feature that numerical nodes failed one at a time rather than in large earthquakes a possible way to remove this unphysical feature from the models would be to include a failure law in which the coefficient of friction increases at first with frictional slip stabilizing the fault and then decreases with further slip destabilizing it

a reevaluation of cosmogenic 36cl production rates in terrestrial rocks we have measured 36cl in a suite of 33 rock samples having well constrained exposure histories and ages the 36cl production parameters were estimated by minimizing the squared deviations between the 36cl and independent ages yielding the following production parameters spallation and muon production from ca 2940 200 atoms 36cl mole ca 1 yr 1 spallation from k 6020 400 atoms 36cl mole k 1 yr 1 and neutron production in air 586 40 fast neutrons g air 1 yr 1 the new production constants for spallation on ca and thermal neutron activation are in good agreement with previous results but that for spallation on k is about 50 larger the past decade has seen a rapid growth in the application of terrestrial cosmogenic nuclides produced by cosmic ray particles within rocks at the earth surface nuclear cross sections particularly for reactions initiated by neutrons and muons are not known with sufficient accuracy so production rates for these nuclides must be empirically calibrated using independently dated geological samples direct production estimates for 36cl were first obtained by zreda et al 1991 who measured 36c1 in mineral separates from granitic boulders on moraine crests at chiatovitch creek in the eastern white mountains of california and nevada this data set was the best available at the time but since then several factors have prompted a reevaluation of the 36cl production rates one is that zreda et al 1994 have shown that there can be significant scatter of the cosmogenic ages of individual glacial boulders toward ages younger than that of the moraine due to soil erosion a second factor is that a much wider range of samples is now available for this purpose finally recent theoretical advances in calculating production by thermal neutron absorption liu et al 1994 now permit more realistic calibration of this reaction

on palaeozoic mesozoic brittle normal faults along the sw barents sea margin fault processes and implications for basement permeability and margin evolution palaeozoic mesozoic brittle normal faults onshore along the sw barents sea passive margin off northern norway give valuable insight into fault and fluid flow processes from the lower brittle crust microstructural evidence suggests that late permian early triassic faulting took place during multiple phases with initial fault movement at minimum p t conditions of c 300 c and c 240 mpa c 10 km depth followed by later fault movement at minimum p t conditions of c 275 c and c 220 mpa c 8 5 km depth the study shows that pore pressures locally reached lithostatic levels 240 mpa during faulting and that faulting came to a halt during early deep stages of rifting along the margin fault permeability has been controlled by healing and precipitation processes through time which have sealed off the core zone and eventually the damage zones after faulting a minimum average exhumation rate of c 40 m ma 1 since the late permian is estimated it implies that the debated late cenozoic uplift of the margin may be explained by increased erosion rates in the coastal regions owing to climate detoriation which caused subsequent isostatic recalibration and uplift of the marginal crust the studied faults may be used as analogues of basement involved fault complexes offshore revealing details about the offshore nature of faulting including past and present basement and fault zone permeability

post 1906 stress recovery of the san andreas fault system calculated from three dimensional finite element analysis 1 the m 7 8 1906 san francisco earthquake cast a stress shadow across the san andreas fault system inhibiting other large earthquakes for at least 75 years the duration of the stress shadow is a key question in san francisco bay area seismic hazard assessment this study presents a three dimensional 3 d finite element simulation of post 1906 stress recovery the model reproduces observed geologic slip rates on major strike slip faults and produces surface velocity vectors comparable to geodetic measurements fault stressing rates calculated with the finite element model are evaluated against numbers calculated using deep dislocation slip in the finite element model tectonic stressing is distributed throughout the crust and upper mantle whereas tectonic stressing calculated with dislocations is focused mostly on faults in addition the finite element model incorporates postseismic effects such as deep afterslip and viscoelastic relaxation in the upper mantle more distributed stressing and postseismic effects in the finite element model lead to lower calculated tectonic stressing rates and longer stress shadow durations 17 74 years compared with 7 54 years all models considered indicate that the 1906 stress shadow was completely erased by tectonic loading no later than 1980 however the stress shadow still affects present day earthquake probability use of stressing rate parameters calculated with the finite element model yields a 7 12 reduction in 30 year probability caused by the 1906 stress shadow as compared with calculations not incorporating interactions the aggregate interaction based probability on selected segments not including the ruptured san andreas fault is 53 70 versus the noninteraction range of 65 77

cosmogenic chlorine 36 from calcium spallation calcium is a major target element for cosmogenic 36cl production consequently 36cl rapidly reaches detectable levels in minerals such as calcite and calcium feldspar exposed at the earth s surface spallation of calcium isotopes typically accounts for 80 90 of 36cl production in these minerals with subsidiary contributions from negative muon capture by 40ca and thermal neutron capture by 35cl to provide a basis for surface exposure dating we have calibrated cosmogenic 36cl production in calcium feldspar from the 17 300 year old tabernacle hill basalt at an altitude of 1445 m and an effective geomagnetic latitude of 40 9 the calcium spallation rate is 152 11 atoms g ca 1 a 1 the corresponding rate at sea level and high latitude is estimated at 48 8 3 4 atoms g ca 1 a 1 the muon capture rate used to derive these values is 8 8 2 2 atoms g ca 1 a 1 at the tabernacle hill site scaled from a value of 4 8 1 2 atoms g ca 1 a 1 at sea level and high latitude the calcium spallation rate determined in this study is in excellent agreement with previous whole rock calibration measurements at tabernacle hill when these are recalculated with respect to the absolute timescale the calibration of 36c1 production from calcium underpins development of an exposure dating technique for calcite due to its high calcium content the 36cl production rate in calcite is higher than in any other common rock forming mineral measurement of 36cl in calcite with an accelerator mass spectrometric detection limit of 5 103 atoms per gram allows dating of limestone surfaces exposed for periods ranging from 102 106 years alternatively erosion rates from less than 1 to greater than 1000 m per year can be determined in the case of eroding karst surfaces though the 36cl production rate is lower in calcium feldspar than in calcite measurements on this mineral will provide a useful means of dating young basalt lavas