collisional snowbird tectonic zone resurrected growth of laurentia during the 1 9 ga accretionary phase of the hudsonian orogeny the 2800 km long snowbird tectonic zone is one of the most controversial tectonic features of the canadian shield metamorphic and in situ geochronologic data reported here reveal that a 1 9 ga medium to high pressure belt extends along most of this tectonic zone in contrast to recent interpretations a collisional origin is indicated by the length of this metamorphic belt tectonic thickening documented in parts of it geological contrasts across it and subduction type microdiamonds within it this collisional event marks a pre 1 865 ga phase of the hudsonian orogeny involving microcontinent accretion that was fundamental to the growth of laurentia

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

alpine and pacific styles of phanerozoic mountain building subduction zone petrogenesis of continental crust a broad continuum exists between two distinct end member types of mountain building alpine type orogenic belts develop during subduction of an ocean basin between two continental blocks resulting in collision they are characterized by an imbricate sequence of oceanward verging nappes some alpine belts exhibit superimposed late stage backthrusting sediments are chiefly platform carbonates and siliciclastics in some cases associated with minor amounts of bimodal volcanics pre existing granitic gneisses and related continental rocks constitute an autochthonous parautochthonous basement metamorphism of deeply subducted portions of the orogen ranges from relatively high pressure hp to ultrahigh pressure uhp calcalkaline volcanic plutonic rocks are rare and have peraluminous s type bulk compositions in contrast pacific type orogens develop within and landward from long sustained oceanic subduction zones they consist of an outboard oceanic trench accretionary prism and an inboard continental margin island arc the oceanic assemblage consists of first cycle in part melanged volcaniclastics and minor but widespread cherts deep water carbonates intimately mixed with disaggregated ophiolites the section recrystallized under hp conditions recumbent fold vergence is oceanward a massive slightly older to coeval calcalkaline arc is sited landward from the trench complex on the stable non subducted plate it consists of abundant dominantly intermediate metaluminous i type volcanics resting on old crust both assemblages are thrown into open folds intruded by comagmatic i type granitoids and metamorphosed locally to regionally under high t low p conditions in the subduction channel of collisional and outboard circumpacific terranes combined extension above and subduction below allows buoyancy driven ascent of ductile thin aspect ratio slices of hp uhp complexes to midcrustal levels where most closely approached neutral buoyancy exposure of rising sheets caused by erosion and gravitational collapse results in moderate amounts of sedimentary debris because exhumed sialic slivers are of modest volume at massive sialic buildups associated with convergent plate cusps syntaxes tectonic aneurysms may help transport hp uhp complexes from mid to upper crustal levels the closure of relatively small ocean basins that typify many intracratonic suture zones provides only limited production of intermediate and silicic melts so volcanic plutonic belts are poorly developed in alpine orogens compared with circumpacific convergent plate junctions generation of a calcalkaline arc mainly depends on volatile evolution at the depth of magma generation phase equilibrium studies show that under typical subduction zone p t trajectories clinoamphibole ca al hydrous silicates constitute the major hydroxyl bearing phases in deep seated metamorphic rocks of morb composition other hydrous minerals are of minor abundance ca and na clinoamphiboles dehydrate at pressures of above approximately 2 gpa but low temperature devolatilization may be delayed by pressure overstepping thus metabasaltic blueschists and amphibolites expel h2o at melt generation depths and commonly achieve stable eclogitic assemblages partly serpentinized mantle beneath the oceanic crust dehydrates at roughly comparable conditions for reasonable subduction zone geothermal gradients however white micas biotites remain stable to pressures 3 gpa accordingly attending descent to depths of 100 km mica rich quartzofeldspathic lithologies that constitute much of the continental crust fail to evolve substantial amounts of h2o and transform incompletely to stable eclogite facies assemblages underflow of amphibolitized oceanic lithosphere thus generates most of the deep seated volatile flux and the consequent partial melting to produce the calcalkaline suite along and above a subduction zone where large volumes of micaceous intermediate and felsic crustal materials are carried down to great depths volatile flux severely diminishes thus continental collision in general does not produce a volcanic plutonic arc whereas in contrast the long continued contemporaneous underflow of oceanic lithosphere does

the 2002 denali fault earthquake alaska a large magnitude slip partitioned event the mw moment magnitude 7 9 denali fault earthquake on 3 november 2002 was associated with 340 kilometers of surface rupture and was the largest strike slip earthquake in north america in almost 150 years it illuminates earthquake mechanics and hazards of large strike slip faults it began with thrusting on the previously unrecognized susitna glacier fault continued with right slip on the denali fault then took a right step and continued with right slip on the totschunda fault there is good correlation between geologically observed and geophysically inferred moment release the earthquake produced unusually strong distal effects in the rupture propagation direction including triggered seismicity

the effects of double fault bends on rupture propagation a geometrical parameter study we use the 2d finite element method to determine how geometrical parameters determine whether rupture will propagate across a linked stepover in a strike slip fault the end segments of the fault system are aligned in the direction of maximum shear and the length and angle of the linking segment are allowed to vary we observe that ruptures propagate through extensional stepovers with steeper angles and longer linking segments than otherwise equivalent compressional step overs these different rupture behaviors form distinct regions in angle stepover length parameter space the boundary between these regions takes the shape of an asymptotic curve in both the extensional and compressional cases models in which the size of the entire fault system was made larger or smaller revealed that the location of the bound aries between regions of different rupture behavior do not scale linearly with the system size it was easier to rupture steeper and relatively longer stepovers in fault systems that were larger overall a separate set of models in which the stress field is rotated so that the parallel end segments were optimally aligned for rupture significantly altered the rupture behavior curves in this stress field it was easier to rupture compressional stepovers with steeper angles and longer linking segments than it was to rupture equivalent extensional stepovers in both the case in which the end segments are aligned with the direction of maximum shear and the case in which the end segments are optimally oriented for rupture the angles at which rupture could no longer propagate through the entire fault corresponded with peaks in the fault s s value

timescales and significance of high pressure high temperature metamorphism and mafic dike anatexis snowbird tectonic zone canada new geochronological isotopic and geochemical data for a spectacular swarm of deep crustal migmatitic mafic dikes offer important insight into processes operative during 1 9 ga high pressure high temperature metamorphism along the snowbird tectonic zone in northern saskatchewan high precision u pb zircon dates reveal anatexis of chipman mafic dikes at 1 896 2 0 3 ma during syntectonic and synmetamorphic intrusion at conditions of 1 0 1 2 gpa 750 c u pb zircon dates of 1 894 1 891 ma for cross cutting pegmatites place a lower bound on major metamorphism and deformation at the currently exposed crustal levels the persistence of elevated temperatures for 14 m y following peak conditions is implied by younger u pb titanite dates and by sm nd whole rock isotopic data that suggest the derivation of the pegmatites by melting of a mafic source limited melting of the host felsic gneiss at 1 9 ga despite high temperature is consistent with evidence for their previous dehydration by granulite facies metamorphism in the archean spatial heterogeneity in patterns of mafic dike and tonalitic gneiss anatexis can be attributed to lateral peak temperature and compositional variability we correlate 1 896 ma chipman mafic dike emplacement and metamorphism with substantial 1 9 ga mafic magmatism over a minimum along strike extent of 1 200 km of the snowbird tectonic zone this suggests a significant continent wide period of asthenospheric upwelling that induced incipient continental rifting extension was subsequently terminated by hinterland contraction associated with trans hudson accretion and orogenesis little activity in the lower crust for ca 650 m y prior to proterozoic metamorphism and mafic magmatism implies an extended interval of cratonic stability that was disrupted at 1 9 ga this episode of destabilization contrasts with the record of long term stability in most preserved cratons and is important for understanding the lithospheric characteristics and tectonic circumstances that control the destruction or survival of continents

tectonic entrapment and its role in the evolution of continental lithosphere an example from the precambrian of western canada new geophysical data acquired over the buried crystalline basement of western canada provide constraints on the history of tectonic assembly of the western canadian shield in the interval 1 75 1 85 ga specifically these data provide new perspectives on the evolution of an archean continental fragment heame province that was trapped in a tectonic vise between coeval orogenic belts that dipped beneath the hearne province the trans hudson orogen developed along the southeastern margin of the hearne province as a series of ocean floor oceanic arc and arc marginal basins were telescoped and thrust obliquely beneath the hearne along the northwest edge of the hearne collapse and subduction of a narrow marginal basin now marked by the subsurface extension of the snowbird tectonic zone led to formation of magmatic arc and collision of older proterozoic terranes the hearne province itself is characterized by regional granulite grade metamorphism and evidence of extensive and pervasive partial melting of the crust the internal character of the hearne province seen on crustal seismic reflection profiles is that of a crustal scale structural fan with reflection fabrics that verge toward the bounding orogens the deformation of the hearne is predominantly of paleoproterozoic age and constitutes a thorough reworking of this formerly archean crustal domain over a distance of more than 600 km across strike entrapment and thermal weakening of the hearne resulted from mechanical coupling of inferred buoyant subduction collision zones and removal of or modification of archean lithospheric mantle that may have originally formed the keel to the hearne long period magnetotelluric profiles show that anomalously conductive mantle lithosphere underlies the present day hearne province which is attributed to metasomatic modification of the subcontinental lithosphere following collisional thickening and delamination convective removal of thickened lithosphere in the proterozoic tectonic entrapment as illustrated by the proterozoic structural and thermal evolution of the hearne province crust and subcontinental mantle may be an example of the lithospheric consequences of opposing collisional polarity during assembly of continents

early deglaciation and paleolake history of rio cisnes glacier patagonian ice sheet 44 s the timing structure and landscape change during the patagonian ice sheet deglaciation remains unresolved in this article we provide a geomorphic stratigraphic and geochronological deglacial record of rio cisnes glacier at 44 s and also from the nearby rio nirehuao and rio el toqui valleys 45 s in chilean patagonia our 14 c 10 be and optically stimulated luminescence data indicate that after the last glacial maximum rio cisnes glacier experienced 100 km deglaciation between 19 0 and 12 3 ka accompanied by the formation of large glacial paleolakes deglaciation was interrupted by several ice readvances and by 16 9 0 3 ka rio cisnes glacier extended only 40 of its full glacial extent the deglaciation of rio cisnes glacier and other sensitive patagonian glaciers occurred at least 1 ka earlier than the ca 17 8 ka normally assumed for the local termination coincident with west antarctic isotope records this early deglaciation can be linked to an orbital forcing driven decline of southern ocean sea ice associated with a distinct atmospheric warming that is apparent for west antarctica through patagonia

dynamic earthquake ruptures in the presence of lithostatic normal stresses implications for friction models and heat production we simulate dynamic ruptures on a strike slip fault in homogeneous and layered half spaces and on a thrust fault in a layered half space with traditional friction models sliding friction exceeds 50 of the fault normal compressive stress and unless the pore pressures approach the lithostatic stress the rupture characteristics depend strongly on the depth and sliding generates large amounts of heat under application of reasonable stress distributions with depth variation of the effective coefficient of friction with the square root of the shear modulus and the inverse of the depth creates distributions of stress drop and fracture energy that produce realistic rupture behavior this ad hoc friction model results in 1 low sliding friction at all depths and 2 fracture energy that is relatively independent of depth additionally friction models with rate weakening behavior which form pulselike ruptures appear to generate heterogeneity in the distributions of final slip and shear stress more effectively than those without such behavior which form cracklike ruptures for surface rupture on a thrust fault the simple slip weakening friction model which lacks rate weakening behavior accentuates the dynamic interactions between the seismic waves and the rupture and leads to excessively large ground motions on the hanging wall waveforms below the center of the fault which are associated with waves radiated to teleseismic distances indicate that source inversions of thrust events may slightly underestimate the slip at shallow depths

dynamic rupture modeling of the transition from thrust to strike slip motion in the 2002 denali fault earthquake alaska we use three dimensional dynamic spontaneous rupture models to investigate the nearly simultaneous ruptures of the susitna glacier thrust fault and the denali strike slip fault with the 1957 m w 8 3 gobi altay mongolia earthquake as the only other well documented case of significant nearly simultaneous rupture of both thrust and strike slip faults this feature of the 2002 denali fault earthquake provides a unique opportunity to investigate the mechanisms responsible for development of these large complex events we find that the geometry of the faults and the orientation of the regional stress field caused slip on the susitna glacier fault to load the denali fault several different stress orientations with oblique right lateral motion on the susitna glacier fault replicate the triggering of rupture on the denali fault about 10 sec after the rupture nucleates on the susitna glacier fault however generating slip directions compatible with measured surface offsets and kinematic source inversions requires perturbing the stress orientation from that determined with focal mechanisms of regional events adjusting the vertical component of the principal stress tensor for the regional stress field so that it is more consistent with a mixture of strike slip and reverse faulting significantly improves the fit of the slip rake angles to the data rotating the maximum horizontal compressive stress direction westward appears to improve the fit even further

fault interactions and large complex earthquakes in the los angeles area faults in complex tectonic environments interact in various ways including triggered rupture of one fault by another that may increase seismic hazard in the surrounding region we model static and dynamic fault interactions between the strike slip and thrust fault systems in southern california we find that rupture of the sierra madre cucamonga thrust fault system is unlikely to trigger rupture of the san andreas or san jacinto strike slip faults however a large northern san jacinto fault earthquake could trigger a cascading rupture of the sierra madre cucamonga system potentially causing a moment magnitude 7 5 to 7 8 earthquake on the edge of the los angeles metropolitan region

terashake2 spontaneous rupture simulations of mw 7 7 earthquakes on the southern san andreas fault abstract previous numerical simulations terashake1 of large m w 7 7 southern san andreas fault earthquakes predicted localized areas of strong amplification in the los angeles area associated with directivity and wave guide effects from northwestward propagating rupture scenarios the terashake1 source was derived from inversions of the 2002 m w 7 9 denali alaska earthquake that source was relatively smooth in its slip distribution and rupture characteristics owing both to resolution limits of the inversions and simplifications imposed by the kinematic parameterization new simulations terashake2 with a more complex source derived from spontaneous rupture modeling with small scale stress drop heterogeneity predict a similar spatial pattern of peak ground velocity pgv but with the pgv extremes decreased by factors of 2 3 relative to terashake1 the terashake2 source excites a less coherent wave field with reduced along strike directivity accompanied by streaks of elevated ground motion extending away from the fault trace the source complexity entails abrupt changes in the direction and speed of rupture correlated to changes in slip velocity amplitude and waveform features that might prove challenging to capture in a purely kinematic parameterization despite the reduced pgv extremes northwest rupturing terashake2 simulations still predict entrainment by basin structure of a strong directivity pulse with pgvs in los angeles and san gabriel basins that are much higher than predicted by empirical methods significant areas of those basins have predicted pgv above the 2 probability of exceedance poe level relative to current attenuation relationships even when the latter includes a site term to account for local sediment depth and wave guide focusing produces localized areas with pgv at roughly 0 1 0 2 poe about a factor of 4 5 above the median in contrast at rock sites in the 0 100 km distance range the median terashake2 pgvs are in very close agreement with the median empirical prediction and extremes nowhere reach the 2 poe level the rock site agreement lends credibility to some of our source modeling assumptions including overall stress drop level and the manner in which we assigned dynamic parameters to represent the mechanical weakness of near surface material future efforts should focus on validating and refining these findings assessing their probabilities of occurrence relative to alternative rupture scenarios for the southern san andreas fault and incorporating them into seismic hazard estimation for southern california

attenuation of radiated ground motion and stresses from three dimensional supershear ruptures 1 radiating shear and rayleigh waves from supershear ruptures form mach waves that transmit large amplitude ground motion and stresses to locations far from the fault we simulate bilateral ruptures on a finite width vertical strike slip fault of width w and half length l with l w breaking the surface of an elastic half space and focus on the wavefield out to distances comparable to l at distances much smaller than w two dimensional plane strain slip pulse models i e models in which the lateral extent of the slip zone is unbounded accurately predict the subsurface wavefield amplitudes in the shear mach wedge of those models are undiminished with distance from the fault when viewed from distances far greater than w rupture is accurately modeled as a moving point source that produces a shear mach cone and on the free surface rayleigh wave mach fronts geometrical spreading of the shear mach cone occurs radially and amplitudes there decrease with the inverse square root of distance the transition between these two asymptotic limits occurs at distances comparable to w similar considerations suggest that rayleigh mach waves suffer no attenuation in the ideally elastic medium studied here the rate at which fault strength weakens at the rupture front exerts a strong influence on the off fault fields only in the immediate vicinity of the fault for both sub rayleigh and supershear ruptures and at the mach fronts of supershear ruptures more rapid weakening generates larger amplitudes at the mach fronts

geochronological constraints on metamorphism magmatism and exhumation of deep crustal rocks of the kramanituar complex with implications for the paleoproterozoic evolution of the archean western churchill province canada u pb and sm nd geochronology establishes an important paleoproterozoic 1 9 ga history for the kramanituar complex located in the interior of the archean western churchill province and provides further insight into the snowbird tectonic zone a crustal scale 2000 km feature whose role during precambrian time remains controversial the kramanituar complex is a window of deep crustal rocks dominated by a granulite facies metagabbroic suite of 1902 ma age with minor supracrustal rocks and charnockite these yield peak equilibrium conditions of 12 15 kbar and 850 900 c which are bracketed between 1910 ma the age of prograde metamorphic monazite in sillimanite and kyanite bearing paragneiss and 1901 ma the cooling ages of titanite and rutile in gabbroic and paragneissic rocks although mineral assemblages reflecting peak metamorphic conditions are widespread some rocks record near isothermal decompression to 8 kbar and 800 c the timing of uplift and exhumation is tightly bracketed between magmatic crystallization of gabbroic anorthosite under granulite facies conditions at 1902 ma and widespread rutile cooling ages of 1901 ma time averaged cooling rates of 100 c ma are estimated for gabbroic anorthosite and leucogranite rocks surrounding the kramanituar complex are mainly amphibolite facies archean plutonic and supracrustal rocks those to the north were at deep crustal conditions coeval with the complex whereas those to the south appear to preserve a record of older mainly neoarchean tectonometamorphic assemblages and fabrics the southern boundary of the complex is inferred to be a normal fault that accommodated tectonic unroofing of the kramanituar complex at 1 9 ga this geochronological data set highlights the protracted history of a segment of the geophysically defined snowbird tectonic zone a structure along which latest significant magmatism and tectonometamorphism has been interpreted as either 2600 or 1800 ma penetrative 1900 ma activity in the east trending chesterfield segment as documented at the kramanituar complex suggests that the complex may have represented a favorably oriented segment of a crustal scale archean fault that was reactivated during paleoproterozoic time possibly in response to collision between the slave and churchill cratons thelon orogen at 1 97 1 9 ga

thermobarometry and geochronology of the uvauk complex a polymetamorphic neoarchean and paleoproterozoic segment of the snowbird tectonic zone nunavut canada the uvauk complex is an ultramylonite bounded granulite facies suite of anorthosite gabbro that forms part of the chesterfield inlet segment of the snowbird tectonic zone following initial anorthosite gabbro magmatism at ca 2 71 ga and a cryptic 2 62 2 60 ga event marked by zircon and monazite growth the uvauk complex experienced two high grade tectonometamorphic events at 2 56 2 50 and 1 91 1 90 ga similar to the 2 56 2 50 ga development of other shear zones in the region the upper amphibolite facies to granulite facies moderately high pressure 8 4 11 0 kbar and 705 760 c 1 kbar 100 mpa m1 event is interpreted to have involved the structural emplacement of ca 2 71 ga uvauk complex rocks on ca 2 68 ga tonalitic rocks to the south granulite facies high pressure 11 2 14 7 kbar and 695 865 c m2 metamorphism gabbroic magmatism and mylonite development within the complex at ca 1 9 ga culminated with 3 5 kbar decompression at high temperature clockwise pressure temperature time p t t

application of trishear fault propagation folding to active reverse faults examples from the dalong fault gansu province nw china abstract determining accurate fault slip rates at 1 ka to 1 ma timescales requires well constrained palinspastic reconstructions of dateable geomorphic and or geologic markers although general kinematic models have been developed to simultaneously reconstruct both bedrock e g bedding and fault attitudes and neotectonic markers e g strath terraces along active strike slip and thrust faults it is not clear if these models can also account for deformation along steeply dipping 45 reverse faults to address this problem we have investigated the active 50 dipping dalong reverse fault system this 40 km long fault system forms part of the aksai restraining stepover along the active left slip altyn tagh fault in northwestern china our geometric and kinematic analyses show that conventional fault bend fold models cannot satisfy the steeply dipping fault geometry we observe in the bedrock record likewise standard fault propagation fold models fail to match our measurements of a set of fluvial terraces however by expanding the trishear model of fault propagation folding to track both bedrock and neotectonic markers we are able to match both sets of records in particular we have developed trishear kinematic models for two sites liuchenzi and qingyazi using the numerical modeling program fault fold v 5 0 this work indicates that an important implication of active trishear fault propagation folding is that terrace deformation extends for over 1 km on either side of the fault trace thus to accurately measure the total magnitude of vertical separation between matching terraces in the hanging wall and footwall terrace profiles across active reverse faults must extend 1 2 km on either side of this zone of deformation

evidence for large earthquakes in metropolitan los angeles the sierra madre fault along the southern flank of the san gabriel mountains in the los angeles region has failed in magnitude 7 2 to 7 6 events at least twice in the past 15 000 years restoration of slip on the fault indicated a minimum of about 4 0 meters of slip from the most recent earthquake and suggests a total cumulative slip of about 10 5 meters for the past two prehistoric earthquakes large surface displacements and strong ground motions resulting from greater than magnitude 7 earthquakes within the los angeles region are not yet considered in most seismic hazard and risk assessments

san andreas fault zone california m 5 5 earthquake history the san andreas fault zone has been a very significant source of major california earthquakes from 1812 to 1906 it generated four major earthquakes of m 7 or larger in two pairs on two major portions of the fault a pair of major earthquakes occurred on the central to southern region where the 1857 faulting overlapped the 1812 earthquake faulting a pair of major earthquakes occurred on the northern region where the 1906 faulting overlapped the 1838 earthquake faulting also earthquakes of m 7 occurred in the san francisco bay area on the hayward fault in 1868 and the santa cruz mountains near loma prieta in 1989 and on the imperial fault near the border with mexico in 1940 the 1838 earthquake s damage effects throughout the bay area from san francisco to santa clara valley and monterey were unequalled by any historical earthquake other than the 1906 event this and numerous strong possible aftershocks during the following 3 years in the san juan bautista vicinity suggest 1838 faulting from san francisco to san juan bautista cycles of seismicity and quiescence were associated with the bay area earthquakes of 1868 1906 and 1989 the 1868 earthquake on the hayward fault was preceded by 12 earthquakes of m 5 5 from 1855 to 1866 within 60 km of the hayward fault and was followed by 13 quiet years the 1906 san andreas fault event was preceded from 1881 to 1903 by 18 earthquakes of m 5 5 and was followed by quiescence with only three earthquakes of m 5 5 until 1954 the bay area has been seismically quiet at the m 5 5 level since the 1989 loma prieta earthquake and its 1990 aftershocks which contrasts with the 10 years before 1989 when five m 5 5 6 2 events occurred the loma prieta earthquake is of similar magnitude to the 1868 hayward event and could be followed by a similarly short quiet period the 1857 earthquake had immediate foreshocks in the lonoak bitterwater region 50 km northwest of parkfield in the northern end zone of the 1857 rupture extending southeast from bitterwater 70 km to parkfield the rate of seismic moment release has decreased with time since 1857 this may reflect the decay with time of the stress loading due to the 9 m 1857 fault displacements 80 km southeast of parkfield and explain why the predicted earthquake which was based on the assumption of regular recurrence of parkfield earthquakes has not yet occurred the extent of the 1812 earthquake fault rupture is not well defined jacoby et al 1988 estimated that it extended 170 km from cajon pass to tejon pass based on this estimate we present the hypothesis that the rupture occurred in two segments in december 1812 the eastern segment generated the 8 december earthquake that damaged san juan capistrano san gabriel san fernando and san buenaventura thirteen days later the western segment ruptured generating the earthquake that damaged san fernando and san buenaventura again as well as santa barbara santa ynez and purisima concepcion

late pleistocene intraplate extension of the central anatolian plateau turkey inferences from cosmogenic exposure dating of alluvial fan landslide and moraine surfaces along the ecemis fault zone here we documented the vertical displacement slip rate extension rate and geochronology of normal faults within a narrow strip along the main strand of the ecemis fault zone the kartal cevizlik and lorut faults are normal faults that have evident surface expression within the strip terrestrial cosmogenic nuclide geochronology revealed that the kartal fault deformed the 104 2 16 5 ka aged alluvial fan surface and the cevizlik fault deformed the 21 9 1 8 ka old moraine and talus fan surfaces our topographic surveys indicated 120 10 m and 13 1 1 4 m surface breaking vertical displacements along the kartal and cevizlik faults respectively accordingly we suggest a 1 15 0 21 mm a 1 slip rate and 0 66 0 12 mm a 1 extension rate for the last 104 2 16 5 ka on the kartal fault and a 0 60 0 08 mm a 1 slip rate and 0 35 0 05 mm a 1 extension rate for the last 21 9 1 8 ka on the cevizlik fault we believe that these structures are an integral part of intraplate crustal deformation in the central anatolia they imply that intraplate structures such as the ecemis fault zone may change their mode through time presently the ecemis fault zone has been deformed predominantly by normal faults the presence of steep preserved fault scarps along the kartal cevizlik and lorut faults point to surface breaking normal faulting away from the main strand and particularly signify that these structures need to be taken into account for regional seismic hazard assessments

uniform california earthquake rupture forecast version 2 ucerf 2 the 2007 working group on california earthquake probabilities wgcep 2007 presents the uniform california earthquake rupture forecast version 2 ucerf 2 this model comprises a time independent poisson process earthquake rate model developed jointly with the national seismic hazard mapping program and a time dependent earthquake probability model based on recent earthquake rates and stress renewal statistics conditioned on the date of last event the models were developed from updated statewide earthquake catalogs and fault deformation databases using a uniform methodology across all regions and implemented in the modular extensible open seismic hazard analysis framework the rate model satisfies integrating measures of deformation across the plate boundary zone and is consistent with historical seismicity data an overprediction of earthquake rates found at intermediate magnitudes 6 5 m 7 0 in previous models has been reduced to within the 95 confidence bounds of the historical earthquake catalog a logic tree with 480 branches represents the epistemic uncertainties of the full time dependent model the mean ucerf 2 time dependent probability of one or more m 6 7 earthquakes in the california region during the next 30 yr is 99 7 this probability decreases to 46 for m 7 5 and to 4 5 for m 8 0 these probabilities do not include the cascadia subduction zone largely north of california for which the estimated 30 yr m 8 0 time dependent probability is 10 the m 6 7 probabilities on major strike slip faults are consistent with the wgcep 2003 study in the san francisco bay area and the wgcep 1995 study in southern california except for significantly lower estimates along the san jacinto and elsinore faults owing to provisions for larger multisegment ruptures important model limitations are discussed

enriched archean lithospheric mantle beneath western churchill province tapped during paleoproterozoic orogenesis ultrapotassic rocks of the christopher island formation baker lake basin were emplaced across an enormous area 240 000 km 2 minimum of the western churchill province ca 1 83 ga these rocks extend across the snowbird zone a geophysical feature postulated by others to represent a paleoproterozoic suture that welded the rae and hearne domains minette dikes and flows of the rae and hearne domains display identical nd 1830 values and incompatible element patterns and thus appear to have originated from a common lithospheric mantle source christopher island nd model ages cluster at 2 8 ga and nd data from one archean lamprophyre and three 2 45 to ca 2 2 ga mafic suites suggest that enriched lithospheric mantle sources beneath both the rae and hearne domains existed well before ca 1 83 ga inconsistent with paleoproterozoic suturing along the snowbird zone in contrast to commonly invoked models that envisage melting of local enriched domains christopher island ultrapotassic rocks appear to have originated from an extensive reservoir we suggest that such a reservoir was created during an archean metasomatic event perhaps owing to flat subduction and that it remained in nearly complete isolation until tapped during paleoproterozoic extension related to squeezing of western churchill crust between flanking wopmay and trans hudson orogens

modeling of postseismic relaxation following the great 1857 earthquake southern california abstract repeated first order triangulation measurements made from 15 monuments in central and southern california are used to explore the structure of the crust and upper mantle beneath southern california as well as to estimate the distribution of slip during the great 1857 earthquake the monuments are divided into two subnetworks with repeated observations covering the respective time intervals from 1884 to 1923 and from 1898 to 1923 the angle changes occurring within these times are modeled as a sum of two contributions postseismic displacements following the 1857 earthquake and plate tectonic loading the postseismic displacements have been calculated using a elastic viscoelastic coupling model that for any fault geometry yields the exact displacements on a spherically stratified earth the southern california crust and upper mantle are modeled as an upper elastic plate underlain by a viscoelastic asthenosphere for several different combinations of structural parameters inversions are performed for the slip distribution of the 1857 earthquake in general the best fits to the data are achieved with a triangular slip distribution an elastic plate thickness of 16 km a crustal thickness of 16 to 33 km and a mantle viscosity of 0 4 to 0 8 10 19 pa s although these inversions identify the best rheological models they lead to observed residuals that are inconsistent with a normally distributed data set a preferred model of slip is constructed that accounts for the budget of total slip during the earthquake cycle and satisfies a normally distributed data set the preferred slip distribution is similar in shape to the mapped surface slip distribution but is greater in amplitude by a factor of 1 5 to 2 in addition the san jacinto fault which has no known surface slip associated with the 1857 earthquake is inferred to have accommodated 1 to 3 m of slip during the 1857 earthquake and appears to be the favored site for tectonic strain accumulation since 1857 inferred fault slip exceeding known surface slip implies either unrecognized secondary faulting or a slip deficit at the surface relative to the slip at depth

deformation during terrane accretion in the saint elias orogen alaska the saint elias orogen of southern alaska and adjacent canada is a complex belt of mountains formed by collision and accretion of the yakutat terrane into the transition zone from transform faulting to subduction in the northeast pacific the orogen is an active analog for tectonic processes that formed much of the north american cordillera and is also an important site to study 1 the relationships between climate and tectonics and 2 structures that generate large to great magnitude earthquakes the yakutat terrane is a fragment of the north american plate margin that is partly subducted beneath and partly accreted to the continental margin of southern alaska interaction between the yakutat terrane and the north american and pacific plates causes significant differences in the style of deformation within the terrane deformation in the eastern part of the terrane is caused by strike slip faulting along the fairweather transform fault and by reverse faulting beneath the coastal mountains but there is little deformation immediately offshore the central part of the orogen is marked by thrusting of the yakutat terrane beneath the north american plate along the chugach saint elias fault and development of a wide thin skinned fold and thrust belt strike slip faulting in this segment may be localized in the hanging wall of the chugach saint elias fault or dissipated by thrust faulting beneath a north northeast trending belt of active deformation that cuts obliquely across the eastern end of the fold and thrust belt superimposed folds with complex shapes and plunging hinge lines accommodate horizontal shortening and extension in the western part of the orogen where the sedimentary cover of the yakutat terrane is accreted into the upper plate of the aleutian subduction zone these three structural segments are separated by transverse tectonic boundaries that cut across the yakutat terrane and also coincide with the courses of piedmont glaciers that flow from the topographic backbone of the saint elias mountains onto the coastal plain the malaspina fault pamplona structural zone separates the eastern and central parts of the orogen and is marked by reverse faulting and folding onshore most of this boundary is buried beneath the western or agassiz lobe of the malaspina piedmont glacier the boundary between the central fold and thrust belt and western zone of superimposed folding lies beneath the middle and lower course of the bering piedmont glacier

local near instantaneously dynamically triggered aftershocks of large earthquakes aftershocks are often triggered by static and or dynamic stress changes caused by mainshocks the relative importance of the two triggering mechanisms is controversial at near to intermediate distances we detected and located 48 previously unidentified large early aftershocks triggered by earthquakes with magnitudes between 7 and 8 within a few fault lengths approximately 300 kilometers during times that high amplitude surface waves arrive from the mainshock less than 200 seconds the observations indicate that near to intermediate field dynamic triggering commonly exists and fundamentally promotes aftershock occurrence the mainshocks and their nearby early aftershocks are located at major subduction zones and continental boundaries and mainshocks with all types of faulting mechanisms normal reverse and strike slip can trigger early aftershocks

temporal evolution of the neoarchean central hearne supracrustal belt rapid generation of juvenile crust in a suprasubduction zone setting abstract new u pb zircon ages are reported for 17 volcanic plutonic and sedimentary rocks from the neoarchean central hearne supracrustal belt in the western churchill province of the canadian shield the central hearne supracrustal belt historically referred to as the rankin ennadai greenstone belt is a major granite greenstone terrain with a strike length of over 700 km this study focused on the central and eastern segments of the belt in the kaminak lake area represents the first detailed chronological study of this extensive greenstone belt the geochronological data indicate rapid formation of crust between 2711 and 2667 ma rocks of the oldest assemblage i formed between 2711 and 2691 ma and consist of mixed tholeiitic and calc alkaline mafic and felsic submarine to subaerial volcanic rocks minor plutonic rocks group 1 are associated with this interval a younger assemblage ii of volcanic rock with significant calc alkaline plutonic rocks group 2 developed between 2686 and 2679 ma a period of deformation d1 locally separates the two assemblages major regional penetrative deformation and metamorphism developed during the latter stages of the plutonic event at ca 2680 ma an extensive belt of metasedimentary rocks and associated iron formation was deposited after 2681 ma based on ages of detrital zircon the youngest events include intrusion of 2666 4 1 1 ma post deformational granite group 3 2659 ma carbonatite can j earth sci 29 1992 896 and deposition of post 2660 ma conglomerates and arenites of possible timiskiming type there is no indication of contributions from mesoarchean crust either in inherited grains in igneous rocks or as detrital grains in sedimentary rocks although the evolutionary sequence has parallels to greenstone belts such as the abitibi belt in terms of the rock types and relative timing of events there are some differences we envision development of the belt in an extensional oceanic suprasubduction environment

reconstruction of a large deep crustal terrane implications for the snowbird tectonic zone and early growth of laurentia the 2800 km long snowbird tectonic zone is a well recognized but still enigmatic feature in the western canadian shield it has been interpreted as a paleoproterozoic continental suture or an archean strike slip fault system but here we suggest that the distinctive geometry of the central snowbird tectonic zone is primarily due to the interaction of crosscutting paleoproterozoic intracontinental thrust and strike slip shear zones having a length of hundreds of kilometers first a major zone of thrust sense shearing coeval with early continent continent collision between the superior and western churchill provinces accommodated uplift of a large exposure of granulite facies lower continental crust younger strike slip shear zones perhaps analogous to asian fault systems behind the himalayan orogen offset the thrust zone thus the current geometry and distribution of deep crustal rocks in this region represent a relatively late stage in the tectonic evolution of the western churchill province rather than an accretionary one earlier structures oriented at a high angle to the snowbird tectonic zone may record the fundamental accretionary history in this part of laurentia

review of crustal architecture and evolution in the ungava peninsula baffin island area connection to the lithoprobe ecsoot transect within the trans hudson orogen in northern quebec and southern baffin island parautochthonous archean basement and paleoproterozoic cover as well as allochthonous accreted paleoproterozoic units are exposed in a series of antiformal culminations and complementary synclinoria the parautochthonous rocks of the superior province margin and two assemblages of accreted paleoproterozoic units ungava and meta incognita crustal terranes define a basic tripartite crustal architecture which is characterized by polyphase deformation and metamorphic histories early structures and mineral assemblages are recognized in all three crustal components and shown to be temporally distinct the earlier structures and assemblages are overprinted by younger elements and mineral assemblages that are related to at least three regional contraction episodes and one thermal event that are common to all crustal components of the orogen close correlation of regional aeromagnetic data in northern quebec and southern baffin island with the three crustal components offers a first order approximation of the distribution of basement lithological units across hudson strait continuation of the aeromagnetic domains beneath ungava bay to northeastern quebec and northern labrador allows for the southward extrapolation of the tripartite trans hudson orogen crustal architecture into the lithoprobe eastern canadian shield onshore offshore transect area

fault kinematic and geomorphic observations along the north tehran thrust and mosha fasham fault alborz mountains iran implications for fault system evolution and interaction in a changing tectonic regime summary neighbouring faults can interact potentially link up and grow and consequently increase the seismic and related natural hazards in their vicinity despite evidence of quaternary faulting the kinematic relationships between the neighbouring mosha fasham fault mff and the north tehran thrust ntt and their temporal evolution in the alborz mountains are not well understood the ene striking ntt is a frontal thrust that delimits the alborz mountains to the south with a 2000 m topographic front with respect to the proximal tehran plain however no large instrumentally recorded earthquakes have been attributed to that fault in contrast the sigmoidally shaped mff is a major strike slip fault located within the alborz mountains sinistral motion along the eastern part of the mff is corroborated by microseismicity and fault kinematic analysis which documents recent transtensional deformation associated with nne ssw oriented shortening to better understand the activity of these faults on different timescales we combined fault kinematic analysis and geomorphic observations to infer the kinematic history of these structures our fault kinematic study reveals an early dextral shear for the ntt and the central mff responsible for dextral strike slip and oblique reverse faulting during nw oriented shortening this deformation regime was superseded by ne oriented shortening associated with sinistral oblique thrusting along the ntt and the central western mff sinistral strike slip motion along subsidiary faults in the central mff segment and folding and tilting of eocene to miocene units in the mff footwall continued thrusting along the ntt took place during the quaternary however folding in the hanging wall and sinistral stream offsets indicate a left oblique component and quaternary strike slip reactivation of the eastern ntt segment close to its termination this complex history of faulting under different stress directions has resulted in a composite landscape with inherited topographic signatures our study shows that the topography of the hanging wall of the ntt reflects a segmentation into sectors with semi independent uplift histories areas of high topographic residuals and apparent high uplift underscore the fault kinematics combined our data suggest an early mechanical linkage of the ntt and mff fault systems during a former dextral transpressional stage caused by nw compression during ne oriented shortening the ntt and mff were reactivated and incorporated into a nascent transpressional duplex the youngest manifestation of motion in this system is sinistral transtension however this deformation is not observed everywhere and has not yet resulted in topographic inversion

significance of stress transfer in time dependent earthquake probability calculations 1 a sudden change in stress is seen to modify earthquake rates but should it also revise earthquake probability data used to derive input parameters permit an array of forecasts so how large a static stress change is required to cause a statistically significant earthquake probability change to answer that question effects of parameter and philosophical choices are examined through all phases of sample calculations drawing at random from distributions of recurrence aperiodicity pairs identifies many that recreate long paleoseismic and historic earthquake catalogs probability density functions built from the recurrence aperiodicity pairs give the range of possible earthquake forecasts under a point process renewal model consequences of choices made in stress transfer calculations such as different slip models fault rake dip and friction are tracked for interactions among large faults calculated peak stress changes may be localized with most of the receiving fault area changed less than the mean thus to avoid overstating probability change on segments stress change values should be drawn from a distribution reflecting the spatial pattern rather than using the segment mean disparity resulting from interaction probability methodology is also examined for a fault with a well understood earthquake history a minimum stress change to stressing rate ratio of 10 1 to 20 1 is required to significantly skew probabilities with 80 85 confidence that ratio must be closer to 50 1 to exceed 90 95 confidence levels thus revision to earthquake probability is achievable when a perturbing event is very close to the fault in question or the tectonic stressing rate is low

relative and absolute timing of gold mineralization along the meliadine trend nunavut canada evidence for paleoproterozoic gold hosted in an archean greenstone belt the meliadine area of the neoarchean rankin inlet greenstone belt is located on the western shore of hudson bay canada and is host to several important orogenic or mesothermal gold deposits supracrustal rocks comprise alternating southeast striking panels of mafic volcanic and clastic sedimentary rocks both of which are host to banded iron formation the dominant structural element is the pyke break a several kilometer wide high strain zone characterized by multiple foliations and regionally important shear zones gold mineralization is spatially associated with these shear zones the maximum age of mineralization is constrained by the observation that auriferous veins crosscut ca 2450 ma mafic dikes and a minimum age is constrained by undeformed postore lamprophyre dikes emplaced at ca 1830 ma highly concordant shrimp determined ages on hydrothermal monazite indicate auriferous veining occurred at ca 1850 ma these relative and absolute timing constraints suggest gold was concentrated in neoarchean supracrustal rocks during paleoproterozoic tectonothermal activity that was widespread in the western churchill province at ca 1850 ma these observations do not preclude the presence of archean gold occurrences in the rankin inlet greenstone belt but no early deformed ores can be linked to auriferous veins introduced at ca 1850 ma suggesting remobilization from earlier formed deposits was not a factor in concentrating gold at meliadine

constraining fault constitutive behavior with slip and stress heterogeneity 1 we study how enforcing self consistency in the statistical properties of the preshear and postshear stress on a fault can be used to constrain fault constitutive behavior beyond that required to produce a desired spatial and temporal evolution of slip in a single event we explore features of rupture dynamics that 1 lead to slip heterogeneity in earthquake ruptures and 2 maintain these conditions following rupture so that the stress field is compatible with the generation of aftershocks and facilitates heterogeneous slip in subsequent events our three dimensional finite element simulations of magnitude 7 events on a vertical planar strike slip fault show that the conditions that lead to slip heterogeneity remain in place after large events when the dynamic stress drop initial shear stress and breakdown work fracture energy are spatially heterogeneous in these models the breakdown work is on the order of mj m 2 which is comparable to the radiated energy these conditions producing slip heterogeneity also tend to produce narrower slip pulses independent of a slip rate dependence in the fault constitutive model an alternative mechanism for generating these confined slip pulses appears to be fault constitutive models that have a stronger rate dependence which also makes them difficult to implement in numerical models we hypothesize that self consistent ruptures could also be produced by very narrow slip pulses propagating in a self sustaining heterogeneous stress field with breakdown work comparable to fracture energy estimates of kj m 2

a three dimensional semianalytic viscoelastic model for time dependent analyses of the earthquake cycle 1 exploring the earthquake cycle for large complex tectonic boundaries that deform over thousands of years requires the development of sophisticated and efficient models in this paper we introduce a semianalytic three dimensional 3 d linear viscoelastic maxwell model that is developed in the fourier domain to exploit the computational advantages of the convolution theorem a new aspect of this model is an analytic solution for the surface loading of an elastic plate overlying a viscoelastic half space when fully implemented the model simulates 1 interseismic stress accumulation on the upper locked portion of faults 2 repeated earthquakes on prescribed fault segments and 3 the viscoelastic response of the asthenosphere beneath the plate following episodic ruptures we verify both the analytic solution and computer code through a variety of 2 d and 3 d tests and examples on the basis of the methodology presented here it is now possible to explore thousands of years of the earthquake cycle along geometrically complex 3 d fault systems

the dynamics of strike slip step overs with linking dip slip faults fault step overs with linking dip slip faults are common features on long strike slip fault systems worldwide it has been noted by various researchers that under some circumstances earthquakes can jump across fault step overs to cascade into large events while under other circumstances rupture is arrested at step overs there is also evidence that fault step overs may be preferential locations for earthquake nucleation the present work uses the 3d finite element method to model the dynamics of strike slip fault systems with step overs and linking dip slip faults i find that the presence of a linking normal or thrust fault greatly increases the ability of earthquake rupture to propagate across the step over leading to a larger event additionally dilational step overs with linking normal faults are more prone to through going rupture than compressional step overs with linking thrust faults this difference is due to the sign of the normal stress increment on the dip slip fault caused by slip on the strike slip segments slip on the strike slip segments causes a negative unclamping normal stress increment on the linking normal fault in a dilational step over whereas the opposite effect occurs on the linking thrust fault in a compressional step over even in cases for which both dilational and compressional step overs can experience through going rupture dilational step overs typically experience higher slip particularly on the linking normal fault in the compressional case rupture nucleation on the linking thrust fault may increase the likelihood of through going rupture compared to nucleation on one of the strike slip segments near the intersections between the fault segments the stress interaction between the fault segments also causes a significant rotation of rake away from that which would be inferred from the regional stress field the results help to emphasize the importance of two way interactions between nearby fault segments during the earthquake rupture process the results also may have implications for the probability of large earthquakes along geometrically complex strike slip fault systems and may help explain why step overs sometimes act as barriers and other times as nucleation locations for large earthquakes

evidence for large earthquakes on the san andreas fault at the wrightwood california paleoseismic site a d 500 to present we present structural and stratigraphic evidence from a paleoseismic site near wrightwood california for 14 large earthquakes that occurred on the southern san andreas fault during the past 1500 years in a network of 38 trenches and creek bank exposures we have exposed a composite section of interbedded debris flow deposits and thin peat layers more than 24 m thick fluvial deposits occur along the northern margin of the site the site is a 150 m wide zone of deformation bounded on the surface by a main fault zone along the northwest margin and a secondary fault zone to the southwest evidence for most of the 14 earthquakes occurs along structures within both zones we identify paleoearthquake horizons using in filled fissures scarps multiple rupture terminations and widespread folding and tilting of beds ages of stratigraphic units and earthquakes are constrained by historic data and 72 14c ages mostly from samples of peat and some from plant fibers wood pine cones and charcoal comparison of the long well resolved paleoseimic record at wrightwood with records at other sites along the fault indicates that rupture lengths of past earthquakes were at least 100 km long paleoseismic records at sites in the coachella valley suggest that each of the past five large earthquakes recorded there ruptured the fault at least as far northwest as wrightwood comparisons with event chronologies at pallett creek and sites to the northwest suggests that approximately the same part of the fault that ruptured in 1857 may also have failed in the early to mid sixteenth century and several other times during the past 1200 years records at pallett creek and pitman canyon suggest that in addition to the 14 earthquakes we document one and possibly two other large earthquakes ruptured the part of the fault including wrightwood since about a d 500 these observations and elapsed times that are significantly longer than mean recurrence intervals at wrightwood and sites to the southeast suggest that at least the southermost 200 km of the san andreas fault is near failure

microprobe monazite geochronology putting absolute time into microstructural analysis abstract high resolution compositional mapping and dating of monazite on the electron microprobe is a powerful addition to microstructural analysis and an increasingly important tool for tectonic analysis microprobe monazite geochronology can be an efficient reconnaissance tool for evaluating metamorphic and deformational age domains but more importantly its in situ nature and high spatial resolution offer an entirely new level of structurally and texturally specific geochronologic data that can be used to put absolute time constraints on p t d paths constrain the rates of metamorphic and deformational processes and provide new links between metamorphism and deformation microprobe geochronology is particularly applicable to three persistent microstructural microtextural problem areas 1 constraining the chronology of metamorphic assemblages 2 constraining the timing of deformational fabrics and 3 interpreting other geochronological results although some monazite generations can be directly tied to metamorphism or deformation at present the most common constraints rely on monazite inclusion relations in porphyroblasts that in turn can be tied to the deformation and or metamorphic history microprobe mapping and dating allow geochronology to be incorporated into the routine microstructural analytical process resulting in a new level of integration of time t into p t d histories

dynamic transfer of rupture across differently oriented segments in a complex 3 d fault system 1 we simulate spontaneous dynamic propagation of rupture across two adjacent fault segments subject to a triaxial compressive stress regime these segments have different orientations and hence different focal mechanisms vertical strike slip and dipping thrust numerical simulations using a biem boundary integral equation method have revealed that under a typical triaxial homogeneous compressive stress regime where the magnitude of the intermediate principal stress lies halfway between those of the maximum and minimum ones ruptures of different focal mechanisms are not likely to occur simultaneously in a single rupture event propagation of rupture from a vertical strike slip fault segment to a pure dip slip normal reverse fault segment is possible only when the stress field is close to uniaxial compression or when the intermediate stress magnitude is close either to the minimum or the maximum one these findings are useful for evaluating possible earthquake scenarios along fault systems with complex 3d geometries

eclogites of the snowbird tectonic zone petrological and u pb geochronological evidence for paleoproterozoic high pressure metamorphism in the western canadian shield eclogite occurs within the southern domain of the east athabasca mylonite triangle in northern saskatchewan situated at the boundary between the archean rae and hearne provinces of the western canadian shield the east athabasca mylonite triangle is a fundamental exposure of the 3 000 km long snowbird tectonic zone the eclogite occurs in association with a variety of lower crustal high pressure granulites that record a complex metamorphic history from 2 6 to 1 9 ga temperatures of the eclogite facies metamorphism are constrained by garnet clinopyroxene exchange thermometry at 920 1 000 c minimum pressure conditions are recorded by the jadeite quartz albite geobarometer at 1 8 2 0 gpa a near isothermal decompression path to granulite facies conditions is inferred from retrograde reaction textures involving the formation of granulite facies assemblages such as orthopyroxene plagioclase and pargasite plagioclase u pb idtims zircon geochronology of the eclogite yields a weighted mean 207pb 206pb date of 1 904 0 0 3 ma which we interpret as the time of peak eclogite facies metamorphism shrimp in situ analyses of metamorphic zircons included within omphacitic clinopyroxene support this interpretation with a weighted mean 207pb 206pb date of 1 905 19 ma inclusion suites of high pressure phases and the petrographic setting of zircon are a direct link between zircon growth and eclogite facies metamorphism zircon from one eclogite sample has older cores that are 2 54 ga which is a minimum age for the emplacement or earliest metamorphism of the gabbroic protolith u pb rutile data indicate slow cooling at 1 c ma below 500 c from 1 88 to 1 85 ga the formation and exhumation of the eclogites at ca 1 9 ga has important implications for the tectonic significance of the snowbird tectonic zone during the paleoproterozoic the eclogites described here are consistent with transport of continental crust to mantle depths during the paleoproterozoic followed by rapid buoyancy driven exhumation to normal lower crustal depths

tsunamis and splay fault dynamics 1 the geometry of a fault system can have significant effects on tsunami generation but most tsunami models to date have not investigated the dynamic processes that determine which path rupture will take in a complex fault system to gain insight into this problem we use the 3d finite element method to model the dynamics of a plate boundary splay fault system we use the resulting ground deformation as a time dependent boundary condition for a 2d shallow water hydrodynamic tsunami calculation we find that if the stress distribution is homogeneous rupture remains on the plate boundary thrust when a barrier is introduced along the strike of the plate boundary thrust rupture propagates to the splay faults and produces a significantly larger tsunami than in the homogeneous case the results have implications for the dynamics of megathrust earthquakes and also suggest that dynamic earthquake modeling may be a useful tool in tsunami research

physics based scenario of earthquake cycles on the ventura thrust system california the effect of variable friction and fault geometry the ventura thrust system in california is capable of producing large magnitude earthquakes geological studies suggest that the fault geometry is complex composed of multiple segments at different dips thrust ramps dipping 30 50 linked with bed parallel decollements dipping 10 these latter types of gently dipping faults form due to preexisting weaknesses in the crust and therefore have different frictional parameters from thrust ramps the faults also experience different stresses because of how stresses are resolved onto the fault planes here we use a two dimensional fault model to assess how geometry and frictional properties of the ramp decollement system should affect the seismic cycle we test velocity strengthening velocity weakening and conditionally stable decollements and in addition explore how the dip angle of the decollement changes the earthquake behavior a velocity strengthening decollement cannot replicate the through going earthquake ruptures that have been inferred for the ventura fault system we therefore suggest that this and other decollements may be better represented using a velocity weakening or conditionally stable response our results show that minor variations in fault geometry produce slip amounts and recurrence intervals that differ only by 10 20 but do not fundamentally alter the types of earthquakes and interseismic slip we conclude that geological constraints on fault geometry are typically sufficient to produce modeled earthquake sequences that are statistically consistent with paleoseismic records however both frictional parameters along the fault and effective normal stress influence earthquake rupture patterns significantly more research is needed to adequately constrain these quantities in order for earthquake rupture models to work as effective predictors of fault behavior

striding athabasca mylonite zone implications for the archean and early proterozoic tectonics of the western canadian shield study of the northern saskatchewan district of mackenzie segment of the snowbird tectonic zone suggests that fragments of relatively stiff mid archean crust possibly arc related have controlled the localization shape and complex kinematics of the multistage striding athabasca mylonite zone during the archean as well as the geometry of the early proterozoic rifted margin of the western churchill continent by the late archean the striding athabasca mylonite zone was located in the interior of the western churchill continent well removed from the contemporaneous plate margins except for the alberta segment the snowbird tectonic zone was not the site of an early proterozoic plate margin we suggest that the geometry of the archean early proterozoic boundary in the western canadian shield represents a jagged continental margin composed of a pair of reentrants defined by rifted and transform segments these segments were inherited from early proterozoic breakup and controlled by the archean structure

early proterozoic ultrahigh pressure metamorphism evidence from microdiamonds microdiamonds from the akluil k minette dykes nunavut canada are similar to diamonds formed in subducted metamorphic rocks high concentrations of unaggregated nitrogen and positive 15 n suggest that the microdiamonds formed within rocks subducted to ultrahigh pressures before being sampled by the minette magma 1 8 billion years ago this ultrahigh pressure metamorphism in north america probably related to the trans hudson orogen about 2 billion years ago extends the occurrence of ultrahigh pressure metamorphism from 0 6 billion years to before 1 8 billion years ago and suggests that phanerozoic type subductions were active by the early proterozoic

probabilities for jumping fault segment stepovers 1 seismic hazard analysis relies heavily on the segmentation of faults the ability of ruptures to break multiple segments has a big impact on estimated hazard current practice for estimating multiple segment breakage relies on panels of experts voting on their opinions for each case here we explore the probability of elastodynamic ruptures jumping segment stepovers in numerical simulations of segmented fault systems we find a simple functional form for the probability of jumping a segment stepover as a function of stepover distance an exponential falloff with distance we suggest this simple parameterization of jumping probabilities combined with sparse observational data to fix the lengthscale parameter as a new approach to estimating multisegment earthquake hazard

a model of the earthquake cycle along the san andreas fault system for the past 1000 years 1 we simulate 1000 years of the earthquake cycle along the san andreas fault system by convolving best estimates of interseismic and coseismic slip with the green s function for a point dislocation in an elastic plate overlying a viscoelastic half space interseismic slip rate is based on long term geological estimates while fault locking depths are derived from horizontal gps measurements coseismic and postseismic deformation is modeled using 70 earthquake ruptures compiled from both historical data and paleoseismic data this time dependent velocity model is compared with 290 present day geodetic velocity vectors to place bounds on elastic plate thickness and viscosity of the underlying substrate best fit models rms residual of 2 46 mm yr require an elastic plate thickness greater than 60 km and a substrate viscosity between 2 1018 and 5 1019 pa s these results highlight the need for vertical velocity measurements developed over long time spans 20 years our numerical models are also used to investigate the 1000 year evolution of coulomb stress stress is largely independent of assumed rheology but is very sensitive to the slip history on each fault segment as expected present day coulomb stress is high along the entire southern san andreas because there have been no major earthquakes over the past 150 300 years animations s1 and s2 of the time evolution of vector displacement and coulomb stress are available as auxiliary material

differentiating simple and composite tectonic landscapes using numerical fault slip modeling with an example from the south central alborz mountains iran 1 the tectonically driven growth of mountains reflects the characteristics of the underlying fault systems and the applied tectonic forces over time fault networks might be relatively static but stress conditions could change and result in variations in fault slip orientation such a tectonic landscape would transition from a simple to a composite state the topography of simple landscapes is correlated with a single set of tectonic boundary conditions while composite landscapes contain inherited topography due to earlier deformation under different boundary conditions we use fault interaction modeling to compare vertical displacement fields with topographic metrics to differentiate the two types of landscapes by successively rotating the axis of maximum horizontal stress we produce a suite of vertical displacement fields for comparison with real landscapes we apply this model to a transpressional duplex in the south central alborz mountains of iran where nw oriented compression was superseded by neotectonic ne compression the consistency between the modeled displacement field and real landforms indicates that the duplex topography is mostly compatible with the modern boundary conditions but might include a small remnant from the earlier deformation phase our approach is applicable for various tectonic settings and represents an approach to identify the changing boundary conditions that produce composite landscapes it may be particularly useful for identifying changes that occurred in regions where river profiles may no longer record a signal of the change or where the spatial pattern of uplift is complex

paleoproterozoic reworking of an archean thrust fault in the hearne domain western churchill province u pb geochronological constraints the nature timing and control of archean crustal architecture on the distribution of proterozoic reworking in western churchill province are investigated in the yathkyed lake area central nunavut the tyrrell shear zone tsz marks the boundary between the northwestern and central hearne subdomains respectively in the hanging wall and footwall metamorphism and deformation in the hanging wall are ca 2 66 2 50 ga two episodes of metamorphism are recorded in a foliated granodiorite in the footwall at 1818 5 2 and 1827 5 6 ma the latter age is interpreted as the time of amphibolite facies metamorphism and deformation in the footwall a crosscutting dyke provides a minimum age of 1818 2 ma for this metamorphism and deformation the tsz is dominated by dextral oblique slip shear fabrics a suite of leucogranite dykes provide a maximum age of 1816 2 ma for dextral shearing dextral deformation is bracketed between 1815 2 and 1811 1 ma the ages of two other granitoid dykes in an area of low d

u pb geochronological constraints on neoarchean tectonism multiple compressional events in the northwestern hearne domain western churchill province canada in the yathkyed lake area the tyrrell shear zone tsz marks the boundary between the northwestern and central hearne subdomains of the western churchill province the tsz is dominated by proterozoic dextral strain but in areas of low dextral strain older shear fabrics are consistent with an earlier thrusting event four syntectonic granite sheets from within low dextral strain zones in the tsz and contiguous shear zones at the base of the yathkyed belt have ages of 2636 2629 2652 8 2665 2639 and 2644 3 ma the yathkyed belt occurs in the hanging wall of these shear zones and comprises a greenschist to amphibolite grade overturned panel that is overlain by an upper amphibolite grade polydeformed panel the geometry is consistent with a thrust imbricated stack two episodes of deformation in the hanging wall are bracket between ca 2660 and 2616 6 4 ma and between 2616 6 4 ma and ca 2 60 ga the yathkyed belt is interpreted to have initially been emplaced as a thick skinned thrust nappe alon

stress interaction between subduction earthquakes and forearc strike slip faults modeling and application to the northern caribbean plate boundary 1 strike slip faults in the forearc region of a subduction zone often present significant seismic hazard because of their proximity to population centers we explore the interaction between thrust events on the subduction interface and strike slip faults within the forearc region using three dimensional models of static coulomb stress change model results reveal that subduction earthquakes with slip vectors subparallel to the trench axis enhance the coulomb stress on strike slip faults adjacent to the trench but reduce the stress on faults farther back in the forearc region in contrast subduction events with slip vectors perpendicular to the trench axis enhance the coulomb stress on strike slip faults farther back in the forearc while reducing the stress adjacent to the trench a significant contribution to coulomb stress increase on strike slip faults in the back region of the forearc comes from unclamping of the fault i e reduction in normal stress due to thrust motion on the subduction interface we argue that although coulomb stress changes from individual subduction earthquakes are ephemeral their cumulative effects on the pattern of lithosphere deformation in the forearc region are significant we use the coulomb stress models to explain the contrasting deformation pattern between two adjacent segments of the caribbean subduction zone subduction earthquakes with slip vectors nearly perpendicular to the caribbean trench axis is dominant in the hispaniola segment where the strike slip faults are more than 60 km inland from the trench in contrast subduction slip motion is nearly parallel to the caribbean trench axis along the puerto rico segment where the strike slip fault is less than 15 km from the trench this observed jump from a strike slip fault close to the trench axis in the puerto rico segment to the inland faults in hispaniola is explained by different distributions of coulomb stress in the forearc region of the two segments as a result of the change from the nearly trench parallel slip on the puerto rico subduction interface to the more perpendicular subduction slip beneath hispaniola the observations and modeling suggest that subduction induced strike slip seismic hazard to puerto rico may be smaller than previously assumed but the hazard to hispaniola remains high

evidence for a supershear transient during the 2002 denali fault earthquake elastodynamic considerations suggest that the acceleration of ruptures to supershear velocities is accompanied by the release of rayleigh waves along the fault from the stress breakdown zone these waves generate a secondary slip pulse trailing the rupture front but manifest almost entirely in ground motion perpendicular to the fault in the near source region we construct a spontaneously propagating rupture model exhibiting these features and use it to explain ground motions recorded during the 2002 denali fault earthquake at pump station 10 located 3 km from the fault we show that the initial pulses on both the fault normal and fault parallel components are due to the supershear stress release on the fault whereas the later arriving fault normal pulses result from the trailing subshear slip pulse on the fault online material mpeg movies of rupture history and ground motion

suture zone geometry along an irregular paleoproterozoic margin the superior boundary zone manitoba canada lithoprobe acquired a 190 km long deep seismic reflection profile across the superior boundary zone at the eastern margin of the paleoproterozoic trans hudson orogen the profile is located 250 km south of a major promontory along the ancient margin of the superior craton and complements earlier profiles closer to the promontory the new data image the remnants of a craton verging thrust belt younger than 1864 ma within the superior boundary zone extending to a maximum depth of 15 km above a shallowly west dipping basal decollement across the surface suture zone a dip reversal to east dipping reflections occurs within the adjacent reindeer zone where a crustal scale imbricate stack is imaged reindeer zone lower crust and upper mantle extend eastward beneath the superior craton margin for 40 100 km preservation of a foreland thrust belt within the superior boundary zone is explained as a result of reduced collisional convergence and subsequent exhumation within a reentrant flanking the thompson promontory

magnetotelluric and teleseismic study across the snowbird tectonic zone canadian shield a neoarchean mantle suture 1 the snowbird tectonic zone stz is a fundamental boundary within canada s western churchill province one of the world s largest yet poorly known fragments of archean crust geophysical data from a collocated magnetotelluric and teleseismic transect across the northeastern segment of the stz provide an image of its subsurface geometry and indicate that it may have been previously mislocated the model suggests that 1 the stz has played a major role in the neoarchean assembly and paleoproterozoic reworking of the western canadian shield 2 it was reactivated in a manner comparable to other crustal scale features such as the kapuskasing zone of the superior province canada and the redbank thrust of the arunta block central australia and 3 it juxtaposes mantle blocks with contrasting geophysical properties revealing a lithosphere scale overlap of the leading edges of the rae and hearne domains the stz thus records plate interactions in the neoarchean comparable in scale with that of modern orogenic belts