effects of prestress state and rupture velocity on dynamic fault branching 1 we consider a mode ii rupture which propagates along a planar main fault and encounters an intersection with a branching fault using an elastodynamic boundary integral equation formulation allowing the failure path to be dynamically self chosen we study the following questions does the rupture initiate along the branch does it continue is the extensional or compressional side most favored for branching does rupture continue on the main fault too failure is described by a slip weakening law for which the strength at any amount of slip is proportional to normal stress our results show that dynamic stresses around the rupture tip which increase with rupture velocity at locations off the main fault plane relative to those on it could initiate rupture on a branching fault as suggested by prior work whether branched rupture can be continued to a larger scale depends on principal stress directions in the prestress state and on rupture velocity the most favored side for branching rupture switches from the extensional to the compressional side as we consider progressively shallower angles of the direction of maximum compressive prestress with the main fault simultaneous rupturing on both faults can be activated when the branching angle is wide but is usually difficult for a narrow branching angle due to strong stress interactions between faults however it can be also be activated by enhanced dynamic stressing when the rupture velocity is very near the rayleigh velocity natural examples seem consistent with the simulations that we present

small scale deformations associated with the 1992 landers california earthquake mapped by synthetic aperture radar interferometry phase gradients the landers earthquake mw 7 3 occurred on june 28 1992 and ruptured nearly 100 km of previously mapped and unmapped faults in the mojave desert we use synthetic aperture radar interferometry insar to examine the cumulative surface deformation between april 24 and august 7 1992 in a 100 100 km region surrounding the northern portion of the earthquake rupture also we introduce a technique for manipulating sar interferograms to extract short wavelength displacement information this technique involves computation and subsequent combination of interferometric phase gradient maps the insar results show significant deformation signatures associated with faults fractures dry lake beds and mountainous regions within 75 100 km of the main rupture using the phase gradient method we are able to extract small scale deformation patterns near the main rupture many of the preexisting mapped faults within 50 km of the main rupture experienced triggered slip these include the old woman lenwood johnson valley west calico and calico faults the insar results also indicate right lateral offsets along secondary fractures trending n ne within the left lateral zone of shear between the main rupture and the johnson valley fault additionally there are interesting interferogram fringe signatures surrounding troy dry lake and coyote dry lake that are related to deformation of dry lake beds

estimating locations and magnitudes of earthquakes in southern california from modified mercalli intensities modified mercalli intensity mmi assignments instrumental moment magnitudes m and epicenter locations of thirteen 5 6 m 7 1 training set events in southern california were used to obtain the attenuation relation mmi 1 64 1 41 m 0 00526 h 2 63 log h where h is the hypocentral distance in kilometers and m is moment magnitude intensity magnitudes m i and locations for five 5 9 m 7 3 independent test events were consistent with the instrumental source parameters fourteen historical earthquakes between 1890 and 1927 were then analyzed of particular interest are the m i 7 2 9 february 1890 and m i 6 6 28 may 1892 earthquakes which were previously assumed to have occurred near the southern san jacinto fault a more likely location is in the eastern california shear zone ecsz these events and the 1992 m 7 3 landers and 1999 m 7 1 hector mine events suggest that the ecsz has been seismically active since at least the end of the nineteenth century the earthquake catalog completeness level in the ecsz is m 6 5 at least until the early twentieth century online material figures and table of empirical mmi site corrections

dynamic processes from plate subduction to intracontinental deformation insights from the tectono sedimentary evolution of the zhaosu tekesi depression in the southwestern chinese tianshan abstract the zhaosu tekesi depression located to the northern margin of the south tianshan orogenic belt stb is an ideal region for investigating paleozoic orogenic processes and later modifications of the southwestern chinese tianshan in this study we employ 2d seismic and borehole data revealing the geological structure and tectonic evolution of the zhaosu tekesi basin and we report new zircon u pb geochronological and hf isotopic data and whole rock elemental and sr nd isotopic analytical results of the late silurian magmatic rocks as well as the whole rock trace element data of the early carboniferous carbonate rocks borehole data wells zc1 and zc2 and seismic imaging suggest that the zhaosu tekesi depression is composed of silurian lower carboniferous permian jurassic upper cretaceous and paleogene strata with six unconformity surfaces c 1 a s 3 p 1 c 1 a p 2 t p 1 j p 2 t k 2 d j and e k 2 d the silurian strata exhibit southward thrust structures and consist of basalt andesite dacite tuff minor muddy limestone and diorite intrusions zircon shrimp u pb dating for the well zc1 diorites yield a crystallization age of 422 ma with positive e hf t values 3 3 10 6 the diorites and andesites are calc alkaline and show enrichment in liles such as th and u and depletion in nb ta and ti relatively high initial 87 sr 86 sr ratios 0 703376 0 706737 and low e nd t values 0 5 to 1 4 and high th yb 0 29 0 35 and ta yb 1 31 1 58 ratios indicating their formation in an active continental margin setting the lower carboniferous strata mainly comprise oolitic limestone bioclastic micritic limestone and interbedded calcareous siltstone high sr ba 1 31 2 33 v sc 10 4 18 6 and low th u 0 01 0 23 ratios suggest that they were potentially deposited in the mixing near shore marine environment these results in combination with previous data indicate that the southwestern chinese tianshan is controlled by northward subduction of the south tianshan ocean during the late silurian to early carboniferous the presence of the lower permian terrigenous strata and rift basin structure constrains the timing of collision between the ctb kyb and the tarim craton as the late carboniferous thrusting reactivation and syn tectonic growth strata in the basin suggest that the southwestern chinese tianshan experienced late cretaceous compression and neogene modifications after the late permian post faulting thermal subsidence triassic peneplanation and jurassic regional subsidence intracontinental subduction due to distant effect is the main tectonic control on the miocene rapid uplift of the tianshan the zhaosu tekesi basin evolution sheds light on dynamic processes of the southwestern chinese tianshan from plate subduction to intracontinental deformation

geologic investigations of a slip gap in the surficial ruptures of the 1992 landers earthquake southern california a 3 km long gap in the dextral surficial rupture of the 1992 mw 7 3 landers earthquake occurs at the north end of a major fault stepover between the johnson valley and homestead valley faults this gap is situated along a segment of the landers rupture that has been modeled geophysically as having a deficit in average slip at depth to better evaluate the nature of the slip gap we document in detail the character and distribution of surficial rupture within it along the gap is a northwest trending thrust fault rupture with an average of less than 1 rn of northeast directed reverse slip and nearly no oblique right slip we interpret this rupture to be limited to the shallow crust of the northern end of the stepover and to have been the secondary result of dextral shear rather than a mechanism of rigid block slip transfer from the landers kickapoo fault a zone of en echelon extensional ruptures also occurs along the slip gap which we interpret as the secondary result of diffuse dextral shear that accommodated less than 0 5 rn of west northwest extension these secondary ruptures represent a discontinuity in the surficial dextral rupture of the landers earthquake which we propose resulted from the lack of a mature fault connection between the johnson valley and homestead valley faults the rupture pattern of the slip gap implies a significant deficit in net surficial slip which compares favorably with some geophysical models aspects of this rupture pattern also suggest a temporal sequence of rupture that compares favorably with geophysical interpretations of the dynamic rupture propagation

final amalgamation of the tianshan and junggar orogenic collage in the southwestern central asian orogenic belt constraints on the closure of the paleo asian ocean abstract the tianshan and junggar orogenic collage occupies the southwestern part of the central asian orogenic belt and was assembled by collision accretion of several continental blocks and island arcs during late paleozoic early mesozoic time following the consumption of the junggar and south tianshan oceanic basins in the western segment of the paleo asian ocean considerable and continuing controversy has surrounded when where and how this orogenic collage was eventually amalgamated which plays a crucial role in understanding the formation of the central asian orogenic belt this review synthesizes the most recent data and geologic evidence that place critical constraints on the closure history of the oceanic domains a comprehensive data compilation indicates that the formation of ophiolites along major sutures in the tianshan and junggar region lasted to the early carboniferous for the kalamaili suture zone to the mid carboniferous for the west junggar region and the north tianshan suture zone and to the late carboniferous for the south tianshan suture zone ca 325 310 ma ultra high pressure metamorphism along the western part of the south tianshan suture occurred nearly synchronously with the collision between the tarim craton and the central tianshan yili block the oceanic closure and collision events were sequentially followed by widespread and intensive a type granitic and bimodal magmatism which commenced in the mid carboniferous along the kalamaili belt and in the latest carboniferous to earliest permian in other regions contemporaneously large scale strike slip shearing took place along major faults subparallel to main sutures with a dominant dextral sense and deformation time since the carboniferous permian transition available data and geological evidence support an eastward propagating scissor like closure of the western segment of the paleo asian ocean along the south tianshan and north tianshan suture zones over a period from the mid to end carboniferous with the south tianshan belt representing the final suturing site in the early permian the tarim mantle plume likely merely affected the southwestern part of the assembled tarim and tianshan region as indicated by regional discrepancy in type and geochemistry of early permian a type granitoids the amalgamation of the tianshan and junggar orogenic collage was associated with the tectonic bending of the kazakhstan orocline leaving a piece of trapped oceanic crust in its middle part beneath the present junggar basin

u pb hf zircon study of two mylonitic granite complexes in the talas fergana fault zone kyrgyzstan and ar ar age of deformations along the fault abstract a 2000 km long dextral talas fergana strike slip fault separates eastern terranes in the kyrgyz tien shan from western terranes the aim of this study was to constrain an age of dextral shearing in the central part of the fault utilizing ar ar dating of micas we also carried out a u pb hf zircon study of two different deformed granitoid complexes in the fault zone from which the micas for ar dating were separated two samples of the oldest deformed neoproterozoic granitoids in the area of study yielded u pb zircon shrimp ages 728 11 ma and 778 11 ma characteristic for the cryogenian bolshoi naryn formation and zircon grains analyzed for their lu hf isotopic compositions yielded ehf t values from 11 43 to 16 73 and their calculated thfc ages varied from 2 42 to 2 71 ga thus varying cryogenian ages and noticeable heterogeneity of meso to paleoproterozoic crustal sources was established for mylonitic granites of the bolshoi naryn formation two samples of mylonitized pegmatoidal granites of the kyrgysh complex yielded identical 206 pb 238 u ages of 279 5 ma corresponding to the main peak of late paleozoic post collisional magmatism in the tien shan seltmann et al 2011 and zircon grains analyzed for their lu hf isotopic compositions yielded ehf t values from 11 43 to 16 73 and calculated thfc ages from 2 42 to 2 71 ga indicating derivation from a paleoproterozoic crustal source microstructural studies showed that ductile brittle deformation of pegmatoidal granites of the kyrgysh complex occurred at temperatures of 300 400 c and caused resetting of the k ar isotope system of primary muscovite deformation of mylonitized granites of the bolshoi naryn formation occurred under high temperature conditions and resulted in protracted growth and recrystallization of micas the oldest ar ar muscovite age of 241 ma with a well defined plateau from a pegmatoidal granite of the kyrgysh complex is considered as a minimum age of dextral motions along this section of the fault in the triassic while younger ages varying from 227 ma to 199 ma with typical staircase patterns indicate protracted growth and recrystallization of micas during ductile deformations which continued until the end of the triassic

relationship of the kickapoo fault to the johnson valley and homestead valley faults san bernardino county california the 1992 mw 7 3 landers earthquake rupture propagated from the johnson valley fault to the homestead valley fault across homestead valley rupture was concentrated along the kickapoo fault a k a landers fault a previously unrecognized north south striking 5 km long fault in homestead valley and along small secondary fault traces east of the kickapoo fault to better understand the role of the kickapoo fault in accommodating slip between the johnson valley fault and the homestead valley fault we performed detailed aerial photograph interpretation and field mapping of alluvial deposits geomorphology and co seismic surface rupture evaluated existing water well data and conducted a gravity survey of the homestead valley area these data show that the kickapoo fault is a pre 1992 structure best interpreted as the northward continuation of the southern johnson valley fault the kickapoo fault merges northward with the northwest striking homestead valley fault thus we interpret that the johnson valley kickapoo and homestead valley faults form an interlinked system large ruptures similar to the 1992 landers rupture may have repeatedly propagated between the johnson valley and homestead valley faults along the kickapoo fault the kickapoo johnson valley fault intersection defines a 1 km wide left restraining bend uplifted alluvial deposits and pressure ridges are common within the bend indicating the bend is a long lived contractional feature homestead valley itself is underlain by relatively thin alluvial deposits over crystalline bedrock showing that no significant subsidence has occurred during the late quaternary these observations are consistent with the accommodation of north south right shear on the southern johnson valley and kickapoo faults in homestead valley

role of fault branches in earthquake rupture dynamics 1 we analyze earthquake ruptures propagating along a straight main fault and encountering a finite length branch fault such intersections are often observed in natural fault systems the predicted effects of the interaction with the branch that we report can be remarkable they can strongly perturb the propagation velocity on the main fault and in some cases even arrest that propagation earlier work kame et al 2003 bhat et al 2004 emphasized the role of the fault pre stress state branch geometry i e branching angle and the incoming rupture velocity at the branching junction in determining whether the rupture would follow the branch or continue on the main fault or both through simulations which did not let a rupture on the branch encounter a barrier or a fault end called infinite branch cases henceforth in this study we look at finite branch cases and study the effect also of branch length with rupture being blocked from propagation beyond the branch end it is known that sudden stoppage of a dynamic rupture front leads to the propagation of large dynamic stress perturbations in the medium these have been known to nucleate or terminate ruptures on adjacent fault segments harris et al 1991 harris and day 1993 1999 harris et al 2002 fliss et al 2005 among others we thus anticipate interaction between the rupture on the main fault and the branched one at two stages when the rupture is propagating on the branch and when it is suddenly blocked at the branch end we show that in general rupture termination on a compressional branch little affects propagation on the main fault compared to the infinite branch cases for branches on the extensional side we show in some cases that whereas an infinite branch would have allowed or stopped rupture propagation on the main fault a finite branch stops or allows propagation on the main fault such results have a dependence on branch length that we document we also illustrate branch related complexities in rupture velocity evolution which could be one of the sources of the high frequency content of strong ground motion record complexities in the slip distribution often associated with a presumed heterogeneous strength distribution along the fault can also be observed when rupture is terminated on a branch

reassessment of continental growth during the accretionary history of the central asian orogenic belt we argue that the production of mantle derived or juvenile continental crust during the accretionary history of the central asian orogenic belt caob has been grossly overestimated this is because previous assessments only considered the palaeozoic evolution of the belt whereas its accretionary history already began in the latest mesoproterozoic furthermore much of the juvenile growth in central asia occurred in late permian and mesozoic times after completion of caob evolution and perhaps related to major plume activity we demonstrate from zircon ages and nd hf isotopic systematics from selected terranes within the caob that many neoproterozoic to palaeozoic granitoids in the accreted terranes of the belt are derived from melting of heterogeneous precambrian crust or through mixing of old continental crust with juvenile or short lived material most likely in continental arc settings at the same time juvenile growth in the caob occurred during the latest neoproterozoic to palaeozoic in oceanic island arc settings and during accretion of oceanic island arc and precambrian terranes however taking together our data do not support unusually high crust production rates during evolution of the caob significant variations in zircon ehf values at a given magmatic age suggest that granitoid magmas were assembled from small batches of melt that seem to mirror the isotopic characteristics of compositionally and chronologically heterogeneous crustal sources we reiterate that the chemical characteristics of crustally derived granitoids are inherited from their source s and cannot be used to reconstruct tectonic settings and thus many tectonic models solely based on chemical data may need re evaluation crustal evolution in the caob involved both juvenile material and abundant reworking of older crust with varying proportions throughout its accretionary history and we see many similarities with the evolution of the sw pacific and the tasmanides of eastern australia

global climate change and active tectonics effective tools for teaching and research abstract multidisciplinary student teams foster innovative research projects that can discern the locations times and styles of late quaternary faulting field studies in the area of the 1992 landers magnitude 7 3 earthquake in southeastern california provide excellent opportunities for teaching and research in climatic and tectonic geomorphology graduate student work begins with descriptions of type sections for mappable units of quaternary alluvium climate change induced aggradation events these are stair stepped flights of alluvial fan and terrace surfaces topographic profiles along faulted stream channels and fans provide estimates of vertical tectonic deformation and insight into the responses of fluvial systems to wrench fault tectonics time lines in this tectonically deforming landscape are the result of global climate changes that caused synchronous pulses of alluviation in the mojave desert at about 125 55 and 10 12 and 8 ka these dated alluvial surfaces record tectonic deformation resulting from cumulative late quaternary surface ruptures tectonic geomorphologic analyses use topographic pedogenic stratigraphic and structural data but do not require expensive and time consuming trenching projects ages of faulted and unfaulted alluvium constrain the dates of two pre 1992 earthquakes the 1992 landers event is not characteristic of earlier faulting because it ruptured through adjacent segments of the emerson fault that previously ruptured in separate events one at 3 1 2 ka and the other before 8 ka styles of tectonic deformation and locations of active faults however may not have changed during the past 125 ka

the thermo tectonic history of the song kul plateau kyrgyz tien shan constraints by apatite and titanite thermochronometry and zircon u pb dating abstract the song kul basin sits on a plateau at the northern and middle kyrgyz tien shan junction it is a lacustrine basin occupied by lake song kul and predominantly developed on igneous basement this basement was targeted for a multi method chronological study to identify the different magmatic episodes responsible for basement formation and to constrain the timing of the development of its present day morphology zircon u pb dating by la icp ms revealed four different magmatic episodes a late cambrian 500 ma island arc system a late ordovician 450 ma subduction related intrusion an early permian 290 ma collisional stage and a middle to late permian 260 ma post collisional magmatic pulse middle to late triassic 200 230 ma titanite fission track ages and late triassic early jurassic 180 210 ma apatite fission track ages and thermal history modeling indicate the song kul basement was already emplaced in the shallow crust at that time an exhumed fossil apatite fission track partial annealing zone is recognized in the bordering song kul mountain ranges the area experienced only minor post early mesozoic denudation the igneous basement was slowly brought to apatite u th he retention temperatures in the late cretaceous palaeogene miocene to present reactivation of the tien shan does not manifestly affect this part of the orogen

structural styles and zircon ages of the south tianshan accretionary complex atbashi ridge kyrgyzstan insights for the anatomy of ocean plate stratigraphy and accretionary processes abstract the anatomy of an ancient accretionary complex has a significance for a better understanding of the tectonic processes of accretionary orogens and complex because of its complicated compositions and strong deformation with a thorough structural and geochronological study of a fossil accretionary complex in the atbashi ridge south tianshan kyrgyzstan we analyze the structure and architecture of ocean plate stratigraphy in the western central asian orogenic belt the architecture of the atbashi accretionary complex is subdivisible into four lithotectonic assemblages some of which are melanges with block in matrix structure 1 north ophiolitic melange 2 high pressure hp ultra high pressure uhp metamorphic assemblage 3 coherent melange assemblage and 4 south ophiolitic melange relationships between main units are tectonic contacts presented by faults the major structures and lithostratigraphy of these units are thrust fold nappes thrusted duplexes and imbricated ocean plate stratigraphy all these rock units are complicatedly stacked in 3 d with the hp uhp rocks being obliquely southwestward extruded detrital zircon ages of meta sediments provide robust constraints on their provenance from the ili central tianshan arc the isotopic ages of the youngest components of the four units are late permian early middle triassic early carboniferous and early triassic respectively we present a new tectonic model of the south tianshan a general northward subduction polarity led to final closure of the south tianshan ocean in the end permian to late triassic these results help to resolve the long standing controversy regarding the subduction polarity and the timing of the final closure of the south tianshan ocean finally our work sheds lights on the use of ocean plate stratigraphy in the analysis of the tectonic evolution of accretionary orogens

three dimensional nonplanar simulation of the 1992 landers earthquake 1 we have simulated the rupture process of the 1992 landers california earthquake using a realistic nonplanar fault geometry including fault bends and branches through a three dimensional boundary integral equation method considering an external loading system due to local tectonics as well as a nonplanar fault structure we could reproduce the characteristic features of this earthquake such as spontaneous rupture transfer between faults pattern of slip distribution and propagation of rupture front without assuming any laterally heterogeneous distribution of parameters we could also reproduce seismic waveforms near the faults especially in the forward direction of rupture propagation where rupture directivity controlled by rupture transfer between faults significantly affects ground motion

late palaeozoic to mesozoic kinematic history of the talas ferghana strike slip fault kyrgyz west tianshan as revealed by 40ar 39ar dating of syn kinematic white mica abstract the nw trending talas ferghana fault tff in kyrgyzstan central asia is one of the largest intracontinental strike slip faults in the world it extends over a distance of more than 2000 km from southern tourghai to western tarim and exhibits a maximum dextral offset of 200 km during the late palaeozoic to present the history of the fault provides important insights for the understanding of the evolution of southern central asia but remains poorly constrained due to lack of reliable geochronological data we present new ar ar ages and structural data from the kyrgyz west tianshan that elucidate the kinematic history of the tff in the palaeozoic and mesozoic 40ar 39ar ages on mylonitic white micas document a deformational history consisting of several episodes a late carboniferous age of 312 4 ma point to initiation of top to the south and dextral transpressional deformation during a metamorphic overprint in precambrian and palaeozoic rocks along the northern compartment of the tff the main phase of dextral motion along the entire fault occurred in the permian as suggested by minimum ages of 260 290 ma obtained at two different locations in the nw and central parts of the tff partial isotopic resetting occurred between 240 and 210 ma and younger ages of the ar ar mineral ages and structural data argue for a major phase of dextral shearing affecting the entire region from the west tianshan to mongolia in the late permian and leading to formation of almost equally spaced major nw trending dextral strike slip faults the uniform character of this deformation indicates that the process of amalgamation in this part of the caob ended prior to the late permian and central asia evolved as a single coherent continental block since that time

large scale porphyry type mineralization in the central asian metallogenic domain a review abstract a series of porphyry type cu mo au deposits including some giant world class deposits such as kounrad aktogai kal makyr oyu tolgoi and chalukou occur in the central asian metallogenic domain camd the outline of the camd almost overlaps with that of the central asian orogenic belt caob which is located between the east europe siberia tarim and north china cratons the camd comprises sparsely exposed tectonic slices of precambrian high grade metamorphic basement rocks that are sandwiched between phanerozoic accretionary complexes and arc rocks according to geological metallogenic geochronological and tectonic setting characteristics the camd comprises three metallogenic provinces 1 the kazakhstan cu au mo 2 the mongolia cu au and 3 the northeast china mo cu metallogenic provinces early paleozoic 490 to 440 ma and late paleozoic 330 to 295 ma large scale porphyry type mineralization in the kazakhstan metallogenic province is intimately associated with the evolution of the central tianshan yili northern tianshan kokchetav chingiz paleozoic arc system which was terminated by the late permian closure of the western segment of the paleo asian ocean in contrast the 370 ma and 240 ma porphyry type mineralization of the mongolia metallogenic province is related to the tuva mongolia arc collage system that was formed during the evolution of the paleo asian ocean in the paleozoic and the mongol okhotsk ocean in the early mesozoic furthermore the northeast china metallogenic province was affected by three distinct geodynamic processes that were associated with the paleo asian mongol okhotsk and paleo pacific oceans during the paleozoic 250 ma the early mesozoic 200 ma and the late mesozoic 200 90 ma respectively except for the early paleozoic cu mineralization at duobaoshan 480 ma and bainaimiao 440 ma the porphyry type mineralization of this metallogenic province is predominantly characterized by widespread mesozoic mo mineralization in general porphyry type deposits of the camd have geological structural metallogenic alteration and fluid characteristics which are consistent either with those related to the oceanic subductional arc settings of the circum pacific metallogenic domain or with those of the continental collisional environment in the tethyan metallogenic domain the involvement of highly oxidized intermediate to felsic magmas is fundamental for the formation of porphyry deposits even though the prevalent mash and slab melting models can be employed to interpret the genesis of some porphyry cu deposits in the camd a here newly suggested model which is based on pre enriched mafic lower crust and the subduction of a relictic mid oceanic ridge is thought to explain the large scale copper mineralization in the balkhash west junggar metallogenic belt of the kazakhstan metallogenic province nevertheless most of the porphyritic suites associated with the mo deposits in the northeast china metallogenic province have relatively depleted mantle like neodymium and hafnium isotopic compositions suggesting that the ore forming granitic magma may have predominantly been derived from juvenile lower crust upwelling of asthenosphere may have triggered the melting of the lower crust during post collisional extension subsequent to the closure of the paleo asian ocean in the triassic the subduction of the paleo pacific ocean in the jurassic and the thinning of the lithosphere caused by rollback of the paleo pacific plate in the cretaceous ancient crustal components were only locally found to have been involved in the magma source of several medium large mo deposits

seismogenic deformation field in the mojave block and implications for tectonics of the eastern california shear zone from the aftershocks of the 1992 landers earthquake we infer the orientation of the principal strain rate axes d 1 d 2 d 3 d 1 lengthening their relative magnitude and the relative spin of fault blocks by using a micropolar continuum model to invert the seismic p and t axes the seismogenic deformation is consistent with the geodetic measurements of the coseismic displacement and with the secular deformation of the central mojave block regionally the aftershock data define two major domains within the central mojave block 1 the western mojave block including the san bernardino mountains and the epicentral area of the big bear earthquake which is characterized by e w d 1 lengthening and n s d 3 shortening and 2 the central mojave block including the landers surface rupture zone which is characterized by nw se d 1 and ne sw d 3 inversion for the principal strain axes of geodetically measured coseismic displacements across the big bear and landers seismogenic zones gives results similar to the aftershock inversions for those areas indicating that the aftershocks accommodate a deformation similar to the main rupture and do not reflect elastic rebound or residual stresses the background seismicity for 1981 to 1991 shows the same characteristic d 1 and d 3 orientations for the two domains indicating that the secular seismogenic strain has the same regional geometry as the 1992 coseismic deformation the micropolar inversion also provides values of the relative vorticity parameter w which reflects a difference between the vorticity of a shearing continuum and the vorticity of fault bounded blocks rotating within tabular seismogenic shear zones the observed fault geometry along the kickapoo fault suggests a pinned block model for the local block rotation that is consistent with the values of w obtained from our inversions we interpret the regional nw se orientation of d 1 in the central mojave block to be characteristic of the dextral eastern california shear zone which transfers approximately 22 of the pacific north american plate motion from the san andreas system to the walker lane belt in eastern california our results and geodetic determinations of the secular shear strain in the central mojave block indicate that the locus of nw dextral shear generally lies between the san bernardino mountains and the pisgah fault

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

source to sink dynamics in the kyrgyz tien shan from the jurassic to the paleogene insights from sedimentological and detrital zircon u pb analyses new insights in the mesozoic geodynamic evolution and related basin range interactions in the kyrgyz tien shan were obtained based on new sedimentological and detrital zircon u pb la icp ms results from jurassic to paleogene sedimentary sequences studied sedimentary sections are located in the fergana and yarkand fergana basins to the west of the talas fergana fault tff and in the issyk kul and ming kush kokomeren basins to the east the ages of the phanerozoic zircon grains found in 18 jurassic to paleogene sandstones can be divided into four groups caledonian 470 390 ma hercynian 315 260 ma triassic 250 210 ma and jurassic 190 160 ma ages the differences in sedimentation pattern and zircon u pb results suggest that the tff played an important role in the distribution of the sedimentary deposits in the mesozoic during the early middle jurassic the tff was active and the strongest subsidence occurred in the yarkand fergana basin the provenance of its sediments can be explained both by local sources close to the tff and by more distal sources to the south west probably stretching into the present day pamir during the late jurassic early cretaceous renewed compression led to the inversion of the yarkand fergana basin around the jurassic cretaceous transition large alluvial fans developing in a semi arid to arid climate traceable around the whole tien shan were also deposited in the fergana basin a mafic sill intruded within these alluvial fans provides an emplacement age of 144 8 ma apatite u pb indicating sub volcanic activity during the early cretaceous during the late cretaceous to early paleogene the area to the west of the tff experienced marine incursions related to the proto paratethys sea the dominance of caledonian ages in the upper cretaceous lower paleogene detrital samples indicates a change in provenance whereby the drainage areas became larger towards low relief land area east of the tff

new structural data on late paleozoic tectonics in the kyrgyz tien shan central asian orogenic belt abstract the tectonic evolution of the central asian orogenic belt caob is characterized by the successive accretion of lithospheric blocks leading to different interpretations about the polarity of subductions during the paleozoic the number of microplates and oceanic basins and the timing of tectonic events this is especially the case in the tien shan area in this paper we propose new structural maps and cross sections of middle and south kyrgyz tien shan mts and sts respectively these cross sections highlight an overall dextral strike slip shear zone in the mts at the crustal scale and a north verging structure in the sts these structures are carboniferous in age and sealed by a late carboniferous conglomerate later overlain by mesozoic and cenozoic deposits the sts exhibits two deformation phases 1 a top to the south normal shearing that can be related to subduction or exhumation dynamics and 2 a top to the north nappe stacking that we link to the late paleozoic collisional events between the mts and the tarim block we propose a new interpretation of the tectonic evolution of the kyrgyz tien shan during the late paleozoic collision this model involves a partitioned collisional deformation in late carboniferous times with an orthogonal collision to the south between the tarim and mts and a strike slip regime to the north along a dextral e w zone located between the mts and the north tien shan kazakh platform the so called nikolaev line the docking of the large tarim craton against the caob corresponds to a collision phase which ended the long lived paleozoic subduction history in the caob and was followed in the ts region by intense strike slip deformation during the permian

earthquake source parameters and fault kinematics in the eastern california shear zone based on waveform data from a profile of aftershocks following the north south trace of the 28 june 1992 landers rupture across the mojave desert we construct a new velocity model for the mojave region that features a thin slow crust using this model we obtain source parameters including depth and duration for each of the aftershocks in the profile and in addition any significant m 3 7 joshua tree landers aftershock between april 1992 and october 1994 for which coherent terrascope data were available in all we determine source parameters and stress drops for 45 significant m w 4 earthquakes associated with the joshua tree and landers sequences using a waveform grid search algorithm stress drops for these earthquakes appear to vary systematically with location with respect to previous seismic activity proximity to previous rupture i e with respect to the landers rupture and with tectonic province in general for areas north of the pinto mountain fault stress drops of aftershocks located off the faults involved with the landers rupture are higher than those located on the fault with the exception of aftershocks on the newly recognized kickapoo landers fault stress drops are moderate south of the pinto mountain fault where there is a history of seismic swarms but no single throughgoing fault in contrast to aftershocks in the eastern transverse ranges and related to the 1992 big bear california sequence landers events show no clear relationship between stress drop and depth instead higher stress drop aftershocks appear to correlate with activity on nascent faults or those that experienced relatively small slip during mainshock rupture

constraint of fault parameters inferred from nonplanar fault modeling 1 we study the distribution of initial stress and frictional parameters for the 28 june 1992 landers california earthquake through dynamic rupture simulation along a nonplanar fault system we find that observational evidence of large slip distribution near the ground surface requires large nonzero cohesive forces in the depth dependent friction law this is the only way that stress can accumulate and be released at shallow depths we then study the variation of frictional parameters along the strike of the fault for this purpose we mapped into our segmented fault model the initial stress heterogeneity inverted by peyrat et al 2001 using a planar fault model simulations with this initial stress field improved the overall fit of the rupture process to that inferred from kinematic inversions and also improved the fit to the ground motion observed in southern california in order to obtain this fit we had to introduce an additional variations of frictional parameters along the fault the most important is a weak kickapoo fault and a strong johnson valley fault

effects of mesoscopic scale fault structure on dynamic earthquake ruptures dynamic formation of geometrical complexity of earthquake faults 1 we develop a new hierarchical earthquake rupture model that takes into account mesoscopic scale fault structure shear branches nucleated on the main fault are specifically assumed as an example of mesoscopic scale fault structure we numerically investigate dynamic formation of fault geometry and its effects on dynamic earthquake rupture process based on this rupture model as long as the length of the main fault is below a certain threshold lm the growth of these branches is shown to be arrested spontaneously soon after their nucleation the spatial distribution of arrested branches is shown to form a self similar geometrical structure this suggests the existence of a simple scaling relationship between small and large events as long as the length of the main fault is below the threshold lm however once the length of the main fault exceeds lm a limited number of branches begin unstable growth and their sizes soon become comparable to that of the main fault in other words mesoscopic scale branches are spontaneously transformed into macroscopic scale ones this finding indicates the critical importance of the consideration of mesoscopic scale fault structure in understanding rupture dynamics once macroscopic scale branches are formed they change the fault geometry which will considerably affect the rupture dynamics the emergence of macroscopic branches suggests that the above mentioned simple scaling relation is never valid above a critical length lm our study thus indicates that relationship between small and large earthquakes is complicated by the spontaneous transformation of a mesoscopic scale fault structure into a macroscopic scale one

geochemical and geochronological studies of granitoid rocks from the western tianshan orogen implications for continental growth in the southwestern central asian orogenic belt abstract the western tianshan orogen in northwestern china is part of the southwestern margin of the central asian orogenic belt caob which represents the largest tectonic assembly of accretionary complexes and the most significant area of phanerozoic continental growth in the world granitoid rocks have a widespread occurrence and occupy about 30 of the whole exposure of the orogen ages obtained for the granitoids vary from 896 ma to 247 ma while initial 87 sr 86 sr ratios vary from 0 70329 to 0 72070 and e nd values from 14 1 to 7 3 the occurrence of neoproterozoic granitic gneisses implies the presence of precambrian basement in this area during oceanic subduction continental growth occurred as a result of several combined processes i e by addition of oceanic crustal melts adakites the intrusion of basaltic magmas derived by partial melting of the metasomatized depleted mantle wedge and the upwelling of granitic magma derived from a mixed source of basaltic magmas and old continental basement however during the post collisional period vertical accretion of underplated juvenile mantle material may have been accomplished during slab breakoff delamination this study suggests a two stage model of continental growth of syn subduction lateral accretion of arc complexes and post collisional vertical accretion of underplated mantle material which may also be relevant for the western segment of the caob in the phanerozoic the reworking and involvement of old crust are more obvious in the western tianshan orogen compared to other parts of the caob comprising the western junggar eastern junggar alataw and altay terranes

tectonic model and fault segmentation of the median tectonic line active fault system on shikoku japan 1 fault segmentation models have been developed by using various types of information about faults including both geological and geophysical data in order to predict the size of future cascade earthquakes in these models however stress conditions within the fault systems have not been considered in sufficient detail major fault systems generally exhibit heterogeneous stress conditions the median tectonic line active fault system mtlafs is the longest and most active arc parallel right lateral strike slip fault system in japan the stress conditions along the mtlafs change from transpression on eastern shikoku island to transtension on kyushu island the change of stress conditions along the fault system is caused by the counterclockwise rotation of the nankai fore arc sliver in response to the relative motion of the philippine sea and eurasian tectonic plates and back arc spreading in the okinawa trough here we propose stress condition segmentation as a concept that deals with the diversity of stress conditions and can be applied in seismic hazard assessment according to this concept the mtlafs is divided into three segments the east shikoku west shikoku and kyushu segments the east shikoku and kyushu segments are under compressional and tensional stress conditions respectively stress in the west shikoku segment which is between the other two segments is transitional from compressional to tensional conditions in particular change in the magnitude of fault normal stresses along strike is one of the key factors that govern the geometry of fault discontinuities rupture propagation and termination and consequently fault segmentation application of our stress condition segmentation concept to the mtlafs suggests that future earthquakes in the west shikoku and kyushu segments will be of lower energy than those in the east shikoku segment

detailed map displacement paleoseismology and segmentation of the enriquillo plantain garden fault in haiti abstract the enriquillo plantain garden fault epgf ruptured several times in haiti producing large historical earthquakes e g 18 october 1751 21 november 1751 and 3 june 1770 their location and lateral extents are poorly known the most devastating one the mw7 0 12 january 2010 earthquake should have been the occasion to constrain the fault kinematics and geometry unfortunately no significant surface rupture was observed here combining lidar data high resolution aerial photographs geological map and observations in the field we revisited the fault map at a large variety of scales to improve the assessment of seismic hazard associated with the epgf this detailed mapping allowed us to target several sites for paleoseismic trenches we carried out paleoseismic investigations in the clonard pull apart basin during which one trench was opened perpendicular to the fault that trench showed stratigraphic evidence for one paleoearthquake radiocarbon dating in a dry and buried stream led to the conclusion that the paleoearthquake might be that of 1770 then based on kilometric scale geometric complexities we showed that the epgf was composed of more than four segments each capable to generate mw 7 earthquakes finally we identified several geologic and geomorphic markers of long term deformation along the fault we found offsets of 40 15 and 8 km from west to east corresponding to slip rates of 9 3 4 6 and 3 mm yr over the early middle pliocene plio pleistocene and pleistocene respectively such a pattern supports the idea of an eastward propagation of the epgf

postseismic rebound in fault step overs caused by pore fluid flow near field strain induced by large crustal earthquakes results in changes in pore fluid pressure that dissipate with time and produce surface deformation synthetic aperture radar sar interferometry revealed several centimeters of postseismic uplift in pull apart structures and subsidence in a compressive jog along the landers california 1992 earthquake surface rupture with a relaxation time of 270 45 days such a postseismic rebound may be explained by the transition of the poisson s ratio of the deformed volumes of rock from undrained to drained conditions as pore fluid flow allows pore pressure to return to hydrostatic equilibrium

the deep structure and reactivation of the kyrgyz tien shan modelling the past to better constrain the present abstract in the tien shan belt cenozoic to active deformation is guided by structures and rheological contrasts partly inherited from the paleozoic variscan orogeny and reactivated by the india asia collision cenozoic deformation net estimates are constrained by lithospheric scale geological cross sections which rely on reconstructions of the paleozoic central asian orogenic belt caob however several geodynamic scenarii have been proposed for its formation including a south or a north dipping subduction which are still debated due to lack of constraints here we designed numerical experiments to test different hypotheses about the initial geometry and the effective rheology of the paleo sutures and especially the one located between the tien shan and the tarim basin where most cenozoic deformation localizes the different geometries of paleozoic structures are used as input variables in the thermo mechanical models which are then run forward in time after a finite amount of shortening the structures that develop self consistently out of the proposed heterogeneities are then compared to the current cenozoic finite and active deformations we find that a crustal south dipping suture zone lying on a resistant lithospheric mantle best explains the localization of the deformation the current geometry of the structures and the moho depth variations using the model results we propose a mechanically consistent depth interpolated crustal cross section of the kyrgyz tien shan which incorporates both geological and geophysical data

shrimp zircon chronology of hp uhp rocks of the makbal metamorphic complex in the northern tien shan kyrgyzstan abstract in this paper we present the first results of sensitive high resolution ion microprobe shrimp u pb zircon dating obtained for the ultra high pressure uhp metamorphic rocks of the makbal metamorphic complex in the northern tien shan kyrgyzstan zircon grains from two hp eclogite samples and one sample of uhp garnet talk chloritoid schist have similar bright rims identified on cathodoluminescence cl images the rims are interpreted as zircon overgrowth during uhp metamorphism dating of zircon rims from the three samples yielded similar ages of 509 7 ma 502 10 ma and 498 7 ma the time span of 510 500 ma is considered as the minimum age of uhp metamorphism of the makbal complex this age is in accordance with middle cambrian faunal age of the kara archa ophiolite in the makbal area it is suggested that the makbal rocks were metamorphosed in the middle cambrian subduction zone dipping north under the northern tien shan microcontinent a deformed granodiorite associated with the makbal metamorphic rocks yielded an age of 514 5 ma indicating development of a magmatic arc simultaneously with the uhp metamorphism cores of zircon grains from the garnet talk chloritoid schist yielded a wide spectrum of ages 1850 800 ma characteristic for detrital zircons of continental origin a depositional age

architecture and mineral deposit settings of the altaid orogenic collage a revised model abstract the altaids are an orogenic collage of neoproterozoic paleozoic rocks located in the center of eurasia this collage consists of only three oroclinally bent neoproterozoic early paleozoic magmatic arcs kipchak tuva mongol and mugodzhar rudny altai separated by sutures of their former backarc basins which were stitched by new generations of overlapping magmatic arcs in addition the altaids host accreted fragments of the neoproterozoic to early paleozoic oceanic island chains and neoproterozoic to cenozoic plume related magmatic rocks superimposed on the accreted fragments all these assemblages host important many world class late proterozoic to early mesozoic gold copper molybdenum lead zinc nickel and other deposits of various types in the late proterozoic during breakup of the supercontinent rodinia the kipchak and tuva mongol magmatic arcs were rifted off eastern europe siberia and laurentia to produce oceanic backarc basins in the late ordovician the siberian craton began its clockwise rotation with respect to eastern europe and this coincides with the beginning of formation of the mugodzhar rudny altai arc behind the kipchak arc these earlier arcs produced mostly cu pb zn vms deposits although some important intrusion related orogenic au deposits formed during arc arc collision events in the middle cambrian and late ordovician the clockwise rotation of siberia continued through the paleozoic until the early permian producing several episodes of oroclinal bending strike slip duplication and reorganization of the magmatic arcs to produce the overlapping kazakh mongol and zharma saur valerianov beltau kurama arcs that welded the extinct kipchak and tuva mongol arcs this resulted in amalgamation of the western portion of the altaid orogenic collage in the late paleozoic its eastern portion amalgamated only in the early mesozoic and was overlapped by the transbaikal magmatic arc which developed in response to subduction of the oceanic crust of the paleo pacific ocean several world class cu mo porphyry cu pb zn vms and intrusion related au mineral camps which formed in the altaids at this stage coincided with the episodes of plate reorganization and oroclinal bending of magmatic arcs major pb zn and cu sedimentary rock hosted deposits of kazakhstan and central asia formed in backarc rifts which developed on the earlier amalgamated fragments major orogenic gold deposits are intrusion related deposits often occurring within black shale bearing sutured backarc basins with oceanic crust after amalgamation of the western altaids this part of the collage and adjacent cratons were affected by the siberian superplume which ascended at the permian triassic transition this plume related magmatism produced various deposits such as famous ni cu pge deposits of norilsk in the northwest of the siberian craton in the early mesozoic the eastern altaids were oroclinally bent together with the overlapping transbaikal magmatic arc in response to the northward migration and anti clockwise rotation of the north china craton the following collision of the eastern portion of the altaid collage with the siberian craton formed the mongol okhotsk suture zone which still links the accretionary wedges of central mongolia and circum pacific belts in the late mesozoic a system of continent scale conjugate northwest trending and northeast trending strike slip faults developed in response to the southward propagation of the siberian craton with subsequent post mineral offset of some metallogenic belts for as much as 70 400 km possibly in response to spreading in the canadian basin india asia collision rejuvenated some of these faults and generated a system of impact rifts

zircon and molybdenite geochronology and geochemistry of the kalmakyr porphyry cu au deposit almalyk district uzbekistan implications for mineralization processes abstract copper gold and molybdenum mineralization of the kalmakyr porphyry deposit in uzbek tien shan occurs as stockworks veinlets and disseminations in the phyllic and k silicate alteration zones developed predominantly in a middle to late carboniferous intrusive complex composed of monzonite and granodiorite porphyry zircon u pb dating yielded an age of 327 2 5 6 ma for the ore hosting monzonite and an age of 313 6 2 8 ma for the ore bearing granodiorite porphyry re os dating of seven molybdenite samples from stockwork and veinlet ores yielded model ages from 313 2 to 306 3 ma with two well constrained isochron ages of 307 6 2 5 ma five stockwork ores and 309 1 2 2 ma five stockwork ores and two veinlet ores respectively these results indicate that cu au mineralization post dated the emplacement of the monzonite started right after the emplacement of the granodiorite porphyry and lasted for ca 7 ma afterward the geochronological and geochemical data suggest that the kalmakyr deposit was formed in a late carboniferous mature magmatic arc setting probably related to the latest subduction process of the turkestan ocean beneath the middle tien shan the e hf t values of zircon grains from the monzonite vary from 11 to 1 7 with an average of 5 1 and those of zircon grains from the granodiorite porphyry range from 5 7 to 1 8 with an average of 2 4 these data indicate that the magma of both monzonite and granodiorite porphyry was derived from partial melting of a thickened lower crust with input of mantle components and variable crustal contamination and that there was more mantle contribution to the formation of the monzonite than the granodiorite porphyry the high rhenium concentrations of molybdenite 98 899 ppm also indicate major mantle contribution of rhenium and by inference ore metals the relatively high eu n eu n values average 0 68 ce 4 ce 3 values average 890 and ce nd values average 36 8 for zircon grains from the granodiorite porphyry than those from the monzonite average eu n eu n 0 33 average ce 4 ce 3 624 average ce nd 3 9 suggest that the magma for the syn mineralization granodiorite porphyry has higher oxygen fugacity than that for the pre mineralization monzonite based on these data it is proposed that while the monzonite was emplaced the oxygen fugacity and volatile contents in the magma were relatively low and ore metals might disperse in the intrusive rock whereas when the granodiorite porphyry was emplaced the oxygen fugacity and volatile contents in the magma were increased favoring copper and gold enrichment in the magmatic fluids the kalmakyr deposit formed from a long lived magmatic hydrothermal system connected with fertile magmatic sources in relation to the subduction of the turkestan ocean beneath the middle tien shan

dynamic 3d simulations of earthquakes on en echelon faults one of the mysteries of earthquake mechanics is why earthquakes stop this process determines the difference between small and devastating ruptures one possibility is that fault geometry controls earthquake size we test this hypothesis using a numerical algorithm that simulates spontaneous rupture propagation in a three dimensional medium and apply our knowledge to two california fault zones we find that the size difference between the 1934 and 1966 parkfield california earthquakes may be the product of a stepover at the southern end of the 1934 earthquake and show how the 1992 landers california earthquake followed physically reasonable expectations when it jumped across en echelon faults to become a large event if there are no linking structures such as transfer faults then strike slip earthquakes are unlikely to propagate through stepover s 5 km wide

relation between continental strike slip earthquake segmentation and thickness of the crust 1 high resolution maps of large continental strike slip earthquake surface ruptures show that they are formed of fault segments these segments are bounded by fault bends step overs or combinations of the two the lowest limit in size for such segments may not be relevant in the understanding of earthquake mechanics as it pertains to the granular properties of fault zones the maximum limit in segment length however is important as it is directly relates to the maximum extent of seismic rupture to measure the length of the segments a new quantitative method based on piecewise linear fitting is developed and is used to automatically retrieve segments from earthquake rupture maps next this approach is tested against a set of ten continental strike slip earthquake ruptures derived from similar high quality maps the test suggests that segments have a maximum length of 18 km independent of regional tectonic setting slip inversions for earthquakes based on seismological and or geodetic data most often are not unique and can show some variability even for one particular event some basic characteristics however such as total moment release or general source geometry seem to persist that are relevant to earthquake mechanics measurements of the maximum horizontal extent of individual slip patches derived from seismic source inversion for strike slip ruptures show that their strike dimension does not increase infinitely with magnitude but instead reaches a maximum value of 25 km these two independent lines of observations complemented by earlier data and analog experiments suggest that it is the thickness of the seismogenic crust that controls the structural scaling of the length of seismic segments and that it is independent of the ultimate size of individual earthquakes

tectonic evolution of the south tianshan orogen and adjacent regions nw china geochemical and age constraints of granitoid rocks geochemical and geochronological evidence was obtained from granitoids of the south tianshan orogen and adjacent regions which consist of three individual tectonic domains the kazakhstan yili plate the central tianshan terrane and the tarim plate from north to south the central tianshan terrane is structurally bounded by the early paleozoic nikolaev line north nalati fault and late paleozoic atbashy inyl chek south nalati qawabulak fault zones against the kazakhstan yili and tarim plates respectively the meta aluminous to weakly peraluminous granitic rocks which are exposed along the kekesu river and the bikai river across the central tianshan terrane have a tholeiitic calc alkaline or high potassium calc alkaline composition i type geochemical trace element characteristics and the y versus rb nb or y versus nb discrimination diagrams favor a continental arc setting for these granitoid rocks shrimp u pb and la icp ms u pb zircon age data indicate that the magmatism started at about 480 ma continued from 460 to 330 ma and ended at about 275 ma the earlier magmatism 470 ma is considered to be the result of a simultaneous southward and northward subduction of the terskey ocean beneath the northern margin of the tarim plate and the kazakhstan yili plate respectively the later magmatism 460 330 ma is related to the northward subduction of the south tianshan ocean beneath the southern margin of the kazakhstan yili central tianshan plate the dataset presented here in conjunction with previously published data support a late paleozoic tectonic evolution of the south tianshan orogen not a triassic one as recently suggested by shrimp u pb zircon dating for eclogites

spatial variability of seismicity parameters in aftershock zones the spatial variability of the b value of the frequency magnitude relationship and the decay rate of aftershocks as described by the p value of the modified omori law is investigated by using dense spatial grids we map out the distribution of b and p values within the landers northridge morgan hill and kobe aftershock sequences considerable spatial variability is found with b values of independent subvolumes ranging from 0 6 to 1 4 and p values ranging from 0 6 to 1 8 these systematic and statistically highly significant differences argue that it is an oversimplification to assign one single p and b value to an aftershock sequence that extends up to 100 km the spatial distribution of these two parameters is compared with the slip distribution during the mainshock suggesting that the areas of largest slip release correlate with high b value regions we hypothesize that the frictional heat created during the event may influence the p value distribution within an aftershock zone while applied shear stress crack density and pore pressure govern the frequency magnitude distribution by investigating the frequency magnitude distribution separately for preseismic and postseismic periods for the morgan hill mainshock we find that only the volume in the vicinity of the highest slip release shows a significant increase in the b value which decays to premainshock values within a year surrounding areas of the aftershock zone show an approximately constant b value with time because the aftershock hazard after a mainshock depends strongly on both the b and p value we propose that aftershock hazard assessment can be improved by taking into account the spatial distribution of the parameters

relationship of the 1999 hector mine and 1992 landers fault ruptures to offsets on neogene faults and distribution of late cenozoic basins in the eastern california shear zone this report examines the hector mine and landers earthquakes in the broader context of faults and fault related basins of the eastern california shear zone ecsz we compile new estimates of total strike slip offset horizontal separation at nearly 30 fault sites based on offset magnetic anomaly pairs we also present a map of the depth to pre cenozoic basement rock thickness of basin filling late cenozoic deposits for the region based on an inversion of gravity and geologic data our estimates of total long term strike slip offsets on faults that slipped during the 1999 hector mine 3 4 km and the 1992 landers earthquakes 3 1 to 4 6 km fall within the 3 to 5 km range of total strike slip offset proposed for most faults of the western ecsz faults having offsets as great as 20 km are present in the eastern part of the ecsz although the landers rupture followed sections of a number of faults that had been mapped as independent structures the similarity in total strike slip offset associated with these faults is compatible with one of the following hypotheses 1 the landers multistrand rupture is a typical event for this linked fault system or 2 this complex rupture path has acted as a coherent entity when viewed over some characteristic multiearthquake cycle the second hypothesis implies that for each cycle slip associated with smaller earthquakes on individual fault segments integrates to a uniform slip over the length of the linked faults with one exception the region surrounding the hector mine and landers ruptures is devoid of deep late cenozoic basins in particular no deep basins are found immediately north of the pinto mountain fault a place where a number of kinematic models for development of the ecsz have predicted basins in contrast some basins exist near barstow and along the eastern part of the ecsz where the model of dokka et al 1998 predicts basins

hercynian post collisional magmatism in the context of paleozoic magmatic evolution of the tien shan orogenic belt abstract the hercynian tien shan tianshan orogen formed during late palaeozoic collision between the karakum tarim and the kazakhstan paleo continents in order to constrain timing of hercynian post collisional magmatism 27 intrusions were sampled for u pb zircon dating along a ca 2000 km long profile in uzbekistan and kyrgyzstan the samples were dated utilizing sensitive high resolution ion microprobe shrimp ii the obtained ages together with previously published age data allowed the timing of hercynian post collisional magmatism to be constrained and interpreted in the context of the paleozoic magmatic evolution of the region apart from hercynian post collisional magmatism two older magmatic episodes have been recognized and the following sequence of events has been established 1 approximately 10 ma after cessation of continuous caledonian magmatism a number of late silurian early devonian intrusions were emplaced in the middle and northern tien shan terranes between 420 and 390 ma the intrusions probably formed in an extensional back arc setting during coeval subduction under the margins of caledonian paleo kazakhstan continent 2 the next relatively short late carboniferous episode of subduction under paleo kazakhstan was registered in the kurama range of the middle tien shan calc alkaline volcanics and granitoids with ages 315 300 ma have distinct metallogenic affinities typical for subduction related rocks and are not found anywhere outside the middle tien shan terrane west of the talas farghona fault 3 the early permian hercynian post collisional magmatism culminated after the closure of the paleo turkestan ocean and affected the whole region across terrane boundaries the post collisional intrusions formed within a relatively short time span between 295 and 280 ma the model for hercynian post collisional evolution suggests that after collision the tien shan was affected by trans crustal strike slip motions which provided suitable conduits for ascending asthenospheric material and heat influx in the crust this produced both granitoid magmas and hydrothermal fluid flow as a result post collisional intrusions and orogenic au deposits known in the region formed coevally and were tectonically controlled 4 between 240 and 220 ma a triassic thermal event affected the region resulting in resetting and growth of new zircon grains which is detected on a regional scale probably the influx of heat into the crust during the triassic was tectonically focused and varied significantly in different terranes in the region under investigation the triassic thermal event was not accompanied by any significant magmatic activity thus after cessation of hercynian post collisional magmatism ca 280 ma ago there was a long magmatically quiet period in the tien shan

deciphering caledonian events timing and geochemistry of the caledonian magmatic arc in the kyrgyz tien shan abstract in the kyrgyz tien shan also known as tian shan in literature the caledonian cambro silurian intrusions comprise an extensive magmatic arc stretching from east to west for more than 1000 km the characteristic feature of the arc is its relatively homogeneous composition of rock types over the whole structure the kichy naryn and djetim intrusions presented in this study are slightly elongated in an east west direction and occupy an area of ca 100 km2 the main rock types are diorite granodiorite and granite geological and geochemical features of the kichy naryn and djetim intrusions demonstrate characteristics of i type granite series rocks of the two intrusions define a continuous high k calc alkaline series diorite and granite of the kichy naryn intrusion yielded early silurian crystallisation ages of 436 2 ma u pb zircon diorite from pebbles in the conglomerate sampled close to the contact of the kichy naryn intrusion yielded a significantly older early ordovician crystallisation age of 466 10 ma the obtained ages of 466 and 436 ma match ages of two major regional magmatic pulses at ca 435 440 and 460 470 ma which took place during continuous subduction from the cambrian to the silurian the amount of granites in the northern tien shan their prolonged history of formation and pronounced i type geochemical characteristics suggest their formation in an andean type active margin environment

architecture and evolution of accretionary orogens in the altaids collage the early paleozoic west junggar nw china understanding the development and evolution of accretionary orogens is crucial for characterizing continental crust growth in time and space in the altaids tectonic collage of central asia conflicting geodynamic models have been proposed about the paleozoic evolution during which juvenile continental crust has been extensively formed this study focuses on west junggar nw china a key region that has not been extensively studied yet a multidisciplinary approach including detrital zircon provenance study geochemistry and field structural analysis provides new constraints on the evolution of three early paleozoic units of west junggar oceanic subduction predominates in early paleozoic time as indicated by ophiolitic melanges fore arc volcaniclastic turbidites and magmatic arc suites however the development of an olistostrome supplied by a sub contemporaneous carbonate platform the occurrence of continental molasse and the chronology and geochemistry of magmatic events reveal interruptions of subduction discrete collisions of volcanic arcs and micro continent are inferred from the tectonic structure of the belt which displays allochthonous units rooted in the suture zone early paleozoic magmatic and sedimentary rocks of west junggar bear some resemblance to contemporaneous analogues in eastern kazakhstan and suggest a lateral connection on the basis of these new results we propose a geodynamic evolution in four steps 1 early to middle ordovician intra oceanic subduction forming island arcs 2 late ordovician tectonic accretion of island arcs against the kazakhstan continental margin 3 early to middle silurian resumption of subduction and active continental margin magmatism and 4 late silurian early devonian micro continent collision this scenario consistent with the formation of the kazakhstan block supports a model of multiple episodic accretions for the altaids tectonic collage rather than a continuous continental margin evolution these results also suggest that in central asia continental crust growth results from alternating vertical and horizontal growth episodes throughout the paleozoic

analysis of coseismic surface displacement gradients using radar interferometryc new insights into the landers earthquake the map of the coseismic displacement field generated by interferometric processing of synthetic aperture radar lsarr images taken before and after the june 28 1992 landers earthquake sequence brings new insights into the nature of deformation caused by these earthquakes we use the interferometric map generated by massonnet et al l1993r to analyze the surface displacement field in the vicinity of the fault trace complexities in the fringe pattern near the fault reflect shorthwavelength variations of the surface rupture and slip distribution and attest to large displacement gradients along two sections of the fault characteristic fringe patterns can be recognized contrasting in density and direction with patterns observed away from the rupture in order to understand the observed fringe patterns we compute synthetic interferograms in three simple cases l1r rigidhbody rotations about a vertical axis l2r about a horizontal axis ltiltr and l3r distributed simple shear the orientation and spatial separation of interferometric fringes predicted by these models help constrain nearhfield deformation and rupture parameters where the kickapoo fault connects with the homestead valley fault the interferogram shows a clear pattern of parallel n20 w fringes separated by about 160 m this pattern and vertical offsets measured along the kickapoo fault suggest that the block between this fault and the johnson valley fault may have been tilted down to the west a 5hkm block lifted by l m on one side would be tilted by an angle of 0 01 l190 radr producing fringes separated by about 160 m parallel to the tilt axis such a tilt parallel to a n20 w direction would account for the gradual northward increase of the vertical slip component observed along the kickapoo fault this tilt may also explain the 1 m of reverse slip observed along the slip gap section of the homestead valley break between the southern end of the johnson valley fault and the eureka peak fault where no surface rupture has been mapped the dense pattern of fringes implies distributed shear probably resulting from fault slip at depth the density and direction of the fringes in the gap are consistent with a righthlateral slip of 1 2 3 8 m on a blind fault locked above the depth of 1 5 2 km such observations of small wavelength features in the sar interferogram bring new insights into the nearhfield displacement gradient and thus on response of the uppermost crust to seismic rupture

permeability structure and co seismic thermal pressurization on fault branches insights from the usukidani fault japan detailed mapping of complex fault zones shows that secondary faults often branch off the principal slip zone however the effect of secondary branch faults on the hydrodynamic beha viour of fault zones has not yet been examined largely because of a lack of hydraulic data and because numerical or analogue modelling of splay faulting is a complex issue this contribution investigates the thermal pressurization process in cases of slip along a principal slip zone and along splay faults branching off the principal displacement zone the study is based on porosity and permeability data presented in this paper from the principal and secondary slip zones of an active clay rich gouge bearing strike slip fault the usukidani fault of sw japan modelling con strained by these data suggests that thermal pressurization is a viable process only as long as the rupture remains located in the central gouge zones or in mature splay fault gouge zones splaying of the rupture into surrounding microbreccias or into immature or newly generated splay faults of higher permeability will release fluid pressure or inhibit the generation of coseismic excess fluid pressures by thermal pressurization the modelling results suggest that secondary fault branches can play a key role in controlling fluid pressurization during faulting hence complete investi gation of active fault zones needs to include secondary faults and their corresponding hydraulic behaviour in order to establish the influence of such structures on earthquake mechanics

late palaeozoic and meso cenozoic tectonic evolution of the southern kyrgyz tien shan constraints from multi method thermochronology in the trans alai turkestan alai segment and the southeastern ferghana basin abstract in this paper we present new geochronological and thermochronological data from the turkestan alai segment of the southern kyrgyz tien shan the ferghana basin transition and the trans alai pamir foreland in the southern and central parts of the alai range towards the alai valley mainly post collisional granitoids were targeted for zircon u pb dating by laser ablation inductively coupled plasma mass spectrometry la icp ms apatite fission track aft and apatite u th sm he ahe thermochronology these granitoids and their thermally metamorphosed contact aureole yielded middle permian zircon crystallization ages of 284 4 to 264 5 ma aft ages both using the traditional external detector approach as well as using la icp ms for these samples are constrained to the miocene and range between 22 3 1 1 and 8 4 0 3 ma while the ahe ages are somewhat younger between 12 5 0 8 and 5 0 0 3 ma this low temperature thermochronological data suggests an early miocene onset of denudation in this area in the tien shan orogen continuing through the miocene and pliocene this denudation is interpreted as a response to reactivation of the mountain belt in the framework of india eurasia convergence and specific to this region of the pamir tien shan interaction on the northern slopes of the alai range at the edge of the ferghana basin a late palaeozoic conglomerate yielded a reset early cretaceous aft age of 118 7 ma pointing to an older denudation event with respect to those identified in the southern and central parts of the alai range a mesozoic sediment sample from the same region only contains a late triassic age component of 223 ma mesozoic and cenozoic age components on the other hand are all detected in the cenozoic sediments of the trans alai range at the transition between the southern alai valley and the pamirs revealing several meso cenozoic denudation events in the adjoining basement source rocks the age components in these detrital apatites are 207 ma 75 ma 18 ma and 3 ma

coseismic surface deformation from air photos the kickapoo step over in the 1992 landers rupture coseismic deformation of the ground can be measured from aerial views taken before and after an earthquake we chose the area of the kickapoo landers step over along the 1992 landers earthquake zone using air photos scale 1 40 000 scanned at 0 4 m resolution two photos acquired after the earthquake are used to assess the accuracy and to evaluate various sources of noise optical distortions film deformation scanning errors or errors in viewing parameters can yield metric bias at wavelength larger than 1 km offset field at shorter wavelength is more reliable and mainly affected by temporal decorrelation of the images induced by changes in radiometry with time temporal decorrelation and resulting uncertainty on offsets are estimated locally from the correlation degree between the images relative surface displacements are measured independently every about 15 m and with uncertainty typically below 10 cm rms the offset field reveals most of the surface ruptures mapped in the field the fault slip is accurate to about 7 cm rms and measured independently every 200 m from stacked profiles slip distribution compares well with field measurements at the kilometric scale but reveals local discrepancies suggesting that deformation is generally although not systematically localized on the major fault zone located in the field this type of data can provide useful insight into the fault zone s mechanical properties our measurements indicate that elastic coseismic strain near the fault zone can be as large as 0 5 10 3 while anelastic yielding was attained for strain in excess of about 1 2 10 3

rejuvenation of ancient micro continents during accretionary orogenesis insights from the yili block and adjacent regions of the sw central asian orogenic belt abstract in the central asian orogenic belt caob whether substantial juvenile additions associated with accretionary orogenesis are preserved is still a pending issue the yili block yb is a micro continent in the western tianshan sw caob voluminous felsic rocks constitute two major belts stretching in the southern and northern margins of the yb we synthetically compile up to date zircon u pb geochronological elemental and nd hf isotopic data for felsic rocks from the yb and adjacent tectonic domains spatially hf nd isotopic mapping unveils an inboard younging trend in hf model age of the yb which indicates relative ancient basement rocks in its northern and southern edges and the most juvenile crust beneath its centre temporally the compiled zircon hf isotopic dataset suggests alternating continental reworking and growth in the yb and adjacent regions during paleozoic times in combination with other evidence we speculate that the paleozoic continental evolution in the yili block and adjacent regions were associated with episodic advancing and retreating subduction of branches of the paleo asian ocean i e north tianshan ocean in the north and south tianshan ocean in the south and the following orogenic collage continental arc type felsic rocks yield two major populations of 460 to 395 ma and 375 to 310 ma respectively implying two epochs of subduction events punctuated by a magmatic lull in the first stage subduction tectonic switch from advancing to retreating subduction took place around 450 ma in the northern yb and 420 ma in the southern yb the second stage subduction was characterized by a period of trench advance 375 to 350 ma in the northern yb and 370 to 350 ma in the southern yb at the initiation and the following early carboniferous trench retreat 350 to 310 ma in the northern yb and 350 to 322 ma in the southern yb associated with the development of back arc basins the final assembly of the western tianshan orogenic collage plausibly occurred during the late carboniferous in the south a hard collision followed the closure of the south tianshan ocean on the contrary the northern margin of the yb was likely collided softly with an immature nascent island arc the current study highlights a crucial link between supra subduction extension triggered by trench retreat slab rollback and the continental growth of the yili block and adjacent regions on a larger scale such a long term rejuvenation process which was characterized by the gradual replacement of old basement by juvenile crust and associated with subduction zone retreat rollback has been documented in some other micro continents of the caob the preservation of juvenile mixed crust requires some continental margins that did not collide with any ancient micro continent or craton i e non collisional or soft collisional margins even until the termination of accretionary orogenesis the development of a series of oroclines is likely the principal mechanism resulting in survival preservation of juvenile mixed crust within the huge fossil orogen

evolution of cambrian and early ordovician arcs in the kyrgyz north tianshan insights from u pb zircon ages and geochemical data abstract geochronological geochemical and structural studies of magmatic and metamorphic complexes within the kyrgyz north tianshan nts revealed an extensive area of early palaeozoic magmatism with an age range of 540 475 ma during the first episode at 540 510 ma magmatism likely occurred in an intraplate setting within the nts microcontinent and in an oceanic arc setting within the kyrgyz terskey zone in the south during the second episode at 500 475 ma the entire nts represented an arc system these two phases of magmatism were separated by an episode of accretionary tectonics of uncertain nature which led to obduction of ophiolites from the kyrgyz terskey zone onto the microcontinent the occurrence of zircon xenocrysts and predominantly negative whole rock nd t values and hf t values of magmatic zircons suggest a continental setting and melting of precambrian continental sources with minor contributions of palaeozoic juvenile melts in the generation of the magmatic rocks the late cambrian to early ordovician 500 475 ma arc evolved mainly on mesoproterozoic continental crust in the north and partly on oceanic crust in the south arc magmatism was accompanied by spreading in a back arc basin in the south where supra subduction ophiolitic gabbros yielded ages of 496 to 479 ma the relative position of the arc and active back arc basin implies that the subduction zone was located north of the arc dipping to the south variably intense metamorphism and deformation in the nts reflect an early ordovician orogenic event at 480 475 ma resulting from closure of the djalair naiman ophiolite trough and collision of the djel tau microcontinent with the northern margin of nts comparison of geological patterns and episodes of arc magmatism in the nts and chinese central tianshan indicate that these crustal units constituted a single early palaeozoic arc and were separated from the tarim craton by an oceanic basin since the neoproterozoic

planation surfaces of the tian shan range central asia insight on several 100 million years of topographic evolution abstract in central asia numerous fragments of planation surfaces are visible within the present day topography however their precise timing of formations is still poorly constrained and it is not clear if they are remnants of a single extensive planation surface or if they represent different planation episodes by reconstructing the landscape evolution of the tian shan region and by analyzing the relations between the planation surfaces preserved within the eastern tarim basin and the sedimentary record we demonstrate that the numerous erosional surfaces preserved within the tian shan range represent different episodes of surfaces genesis these erosion events span from the late paleozoic to the early cenozoic the widespread preservation of large fragments of these surfaces within the tian shan range implies that this region did not undergo strong relief building during most of its mesozoic evolution but was dominated by plains associated to small hills along episodically active discrete tectonic structures finally the preservation of these surfaces within the active tian shan range implies a long term strong non equilibrium state of the topography during its cenozoic evolution this was probably promoted by the arid to semi arid climate prevailing since the late paleogene onset of relief building

crustal stress map of iran insight from seismic and geodetic computations we used the focal mechanisms of crustal earthquakes depth 40 km in the period 1909 2012 and the available gps velocities derived from the data collected between 1999 to 2011 to estimate the magnitude and directions of maximum principal stress and strain rates in iran the pearson product moment correlation was used to find the correlation between the stress field obtained from the focal mechanism stress inversion and that obtained using the seismic and geodetic strain rates our assumption is that stresses in a continuum are produced by tectonic forces and the consequent deformation on the crustal scale therefore the direction of the stress and strain or strain rate are ideally to be the same our results show a strong correlation between the directions of the principal components of stress and strain rate obtained using the different data methods using weighted average analysis we present a new stress map for iran

dynamic path selection along branched faults experiments involving sub rayleigh and supershear ruptures building upon previous laboratory earthquake experiments of dynamic shear rupture growth taking place along faults with simple kinks new and complex fault geometries involving cohesively held fault branches are studied asymmetric impact at the specimen boundaries controls the incoming shear ruptures which are manipulated to propagate at either sub rayleigh or supershear velocities high speed photography and dynamic photoelasticity are used with a model material homalite 100 to monitor incoming and outgoing rupture propagation acceleration deceleration or arrest at the vicinity of the branch location differences and similarities of rupture velocity history between cases involving faults with either simple kinks or branches on the one hand and sub rayleigh and supershear incoming ruptures on the other are highlighted and explained results of the experiments show a clear general bias toward large branch inclination smaller branch angles appearing to be overshadowed and suppressed by the stress field associated with the main fault of great interest also is the sustenance of rupture propagation along a branch by the mach cone when the initial rupture is supershear driven generally higher rupture speeds favors larger arrays of branching angles to be triggered a companion analysis by templeton et al 2009 featuring detailed numerical simulations of these experiments provides further insight into the observed phenomena

interactions between the landers and hector mine california earthquakes from space geodesy boundary element modeling and time dependent friction the m w 7 1 hector mine earthquake happened 7 years after the 1992 m w 7 3 landers earthquake within a system of right lateral strike slip faults comprising the eastern california shear zone because estimates of recurrence intervals of m 7 earthquakes on these faults range anywhere from 1500 to 50 000 years the close temporal spacing of these two earthquakes suggests that there is some interaction between the two current models of simple coulomb static stress interactions between the earthquake ruptures do not predict an obvious cause and effect relationship the interaction between the computed normal and right lateral shear stress reductions induced by the landers earthquake at the hector mine hypocenter is unclear we use a combination of space geodesy boundary element modeling and computer modeling of time dependent fault friction to investigate the interaction between the two earthquakes we compare stress changes on the hector mine rupture plane induced by the landers earthquake with the detailed slip distribution inferred using a combination of gps and insar data the slip distributions of both earthquakes are also used to infer the magnitude of the shear stress drop on each earthquake rupture and the orientation of the remote background stress consistent with these two recent events for each earthquake the azimuth of the remote maximum compressive stress was approximately 17 6 and the magnitude of the shear stress drop was 8 1 mpa on the landers rupture and 10 2 mpa on the hector mine rupture using a simple spring and slider model as a proxy for the rate and state frictional response of the hector mine faults to landers earthquake imposed stress steps we find that while a decrease in normal stress at the hector mine hypocenter would by itself have caused the hector mine earthquake to nucleate immediately the simultaneous decrease in shear stress may have caused a delay in the peak shear loading consideration of an acceptable range of rate and state friction parameters secular strain rates and crustal stiffnesses leads us to conclude that this delay could have been 0 to 40 years

surface ruptures on the transverse xiaoyudong fault a significant segment boundary breached during the 2008 wenchuan earthquake china abstract the 220 km long rupture of the 2008 mw 7 9 wenchuan earthquake breached several km scale geometric discontinuities along strike including the previously un mapped nw trending xiaoyudong fault connecting between the two major ne trending rupture planes on the beichuan and pengguan faults in this paper we present high resolution mapping of the 8 km long surface breaks and sinistral oblique thrusting coseismic slip on the xiaoyudong fault scarp height is the largest at the nw end reaching 3 5 m and decreases southward in steps to less than 0 2 m with an average slip gradient of 6 10 3 at a few tens of meters length scale but up to 50 10 3 locally left lateral offsets co vary with the vertical component the largest sinistral slip vector we observed is 2 2 m geological and geophysical evidence suggests that the xiaoyudong fault is likely a 30 sw dipping lateral ramp that soles into the pengguan fault and at its northwestern end intersects with the beichuan fault where the latter has a step in the fault plane kinematically the xiaoyudong fault functions as a tear and conjugate fault and coincides with significant coseismic slip rake rotations on both the beichuan and pengguan faults similar correlation of fault bends with sharp changes in faulting style occurs at other steps along the wenchuan rupture the xiaoyudong fault may have played a positive role in linking coseismic slip partitioning between parallel reverse fault planes facilitating the growth of a longer and more destructive rupture this highlights the role of tear faults in bridging ruptures between segments such that reverse type ruptures can breach steps wider than anticipated from strike slip fault examples transfer faults are common and perhaps poorly documented features in reverse fault systems and their roles in ruptures may increase the maximum potential earthquake magnitude for fold and thrust belts

applying wedge theory to dynamic rupture modeling of fault junctions wedges such as accretionary prisms and thin skinned fold and thrust belts occur frequently in nature and can be the site of devastating earthquakes critical wedge theory can be applied to these settings but this steady state description of wedge deformation is at odds with the periodic occurrence of earthquakes we discuss how critical wedge theory applies to the seismic cycle and we use elastic wedge theory to constrain realistic stress states our goal is to determine the rupture behavior of an earthquake in a wedge if rupture initiates on the basal sliding surface will it stay confined to the basal surface or will it propagate onto a fault branch interior to the wedge this information can significantly alter the seismic hazard in areas where fault intersections occur we answer this question using numerical models of dynamic rupture propagation through branched geometries for which the stress state is a pivotal input parameter we apply wedge theory to constrain the stress state but inherent to this theory is the assumption of a weak basal fault we investigate the role of this assumption and determine that rupture is unlikely to propagate from a weak basal fault onto a strong branch fault without the aid of a physical process such as pore fluid migration along the branch this framework is applied to the rupture of the 2008 wenchuan earthquake we find that we are able to reproduce the behavior at some fault intersections but our 2d model is not able to reproduce all the behaviors possibly due to the oblique nature of this event

dynamic rupture propagation on fault planes with explicit representation of short branches abstract an active fault zone is home to a plethora of complex structural and geometric features that are expected to affect earthquake rupture nucleation propagation and arrest as well as interseismic deformation simulations of these complexities have been largely done using continuum plasticity or scalar damage theories in this paper we use a highly efficient novel hybrid finite element spectral boundary integral equation scheme to investigate the dynamics of fault zones with small scale pre existing branches as a first step towards explicit representation of anisotropic damage features in fault zones the hybrid computational scheme enables exact near field truncation of the elastodynamic field allowing us to use high resolution finite element discretization in a narrow region surrounding the fault zone that encompasses the small scale branches while remaining computationally efficient our results suggest that the small scale branches may influence the rupture in ways that may not be realizable in homogenized continuum models specifically we show that these short secondary branches significantly affect the post event stress state on the main fault leading to strong heterogeneities in both normal and shear stresses and also contribute to the enhanced generation of high frequency radiation the secondary branches also affect off fault plastic strain distribution and suggest that co seismic inelasticity is sensitive to pre existing damage features we discuss our results in the larger context of the need for modeling earthquake ruptures with high resolution fault zone physics

paleoseismology of the johnson valley kickapoo and homestead valley faults clustering of earthquakes in the eastern california shear zone paleoseismic data from 11 trenches at seven sites excavated across the southern johnson valley kickapoo and homestead valley faults that ruptured in the 1992 landers earthquake as well as the northern johnson valley fault which did not fail in 1992 indicate that the return period for large surface rupturing events in this part of the eastern california shear zone is in the range of 5 15 ka the inferred slip rates based on their respective recurrence intervals are in the range of 0 2 0 6 mm yr for each of the faults studied a previous large earthquake ruptured the southern johnson valley and kickapoo faults about 5 ka b p the northern johnson valley fault also failed at about this time at 5 8 ka b p and may have been part of the same rupture in contrast the penultimate large earthquake that we identify on the homestead valley fault occurred about 15 ka b p much earlier than other faults involved in the 1992 rupture from these observations combined with paleoseismic work by others after the 1992 earth quake it appears that previous events along the southern johnson valley and kick apoo faults were different than those of 1992 and may have involved other fault segments it has been over 5 ka since the most recent rupture on the northern johnson valley fault therefore it is surprising that it did not fail in the 1992 rupture from our observations dextral shear appears to be distributed across the entire eastern california shear zone with individual faults taking only a small proportion of the overall slip release of this regional strain appears to occur in temporal clusters of large earthquakes with the 1992 event apparently the most recent of a sequence of late holocene 0 1 ka earthquakes that have ruptured the nine faults we have trenched in the southwestern mojave desert previous clusters of earthquake activity occurred in the early 8 9 ka and middle 5 6 ka holocene and possibly the latest pleistocene 15 ka

the 1992 landers earthquake sequence seismological observations the m w 6 1 7 3 6 2 1992 landers earthquakes began on april 23 with the m w6 1 1992 joshua tree preshock and form the most substantial earthquake sequence to occur in california in the last 40 years this sequence ruptured almost 100 km of both surficial and concealed faults and caused aftershocks over an area 100 km wide by 180 km long the faulting was predominantly strike slip and three main events in the sequence had unilateral rupture to the north away from the san andreas fault the m w6 1 joshua tree preshock at 33 n58 and 116 w19 on 0451 ut april 23 was preceded by a tightly clustered foreshock sequence m 4 6 beginning 2 hours before the mainshock and followed by a large aftershock sequence with more than 6000 aftershocks the aftershocks extended along a northerly trend from about 10 km north of the san andreas fault northwest of indio to the east striking pinto mountain fault the m w7 3 landers mainshock occurred at 34 n13 and 116 w26 at 1158 ut june 28 1992 and was preceded for 12 hours by 25 small m 3 earthquakes at the mainshock epicenter the distribution of more than 20 000 aftershocks analyzed in this study and short period focal mechanisms illuminate a complex sequence of faulting the aftershocks extend 60 km to the north of the mainshock epicenter along a system of at least five different surficial faults and 40 km to the south crossing the pinto mountain fault through the joshua tree aftershock zone towards the san andreas fault near indio the rupture initiated in the depth range of 3 6 km similar to previous m 5 earthquakes in the region although the maximum depth of aftershocks is about 15 km the mainshock focal mechanism showed right lateral strike slip faulting with a strike of n10 w on an almost vertical fault the rupture formed an arclike zone well defined by both surficial faulting and aftershocks with more westerly faulting to the north this change in strike is accomplished by jumping across dilational jogs connecting surficial faults with strikes rotated progressively to the west a 20 km long linear cluster of aftershocks occurred 10 20 km north of barstow or 30 40 km north of the end of the mainshock rupture the most prominent off fault aftershock cluster occurred 30 km to the west of the landers mainshock the largest aftershock was within this cluster the m w6 2 big bear aftershock occurring at 34 n10 and 116 w49 at 1505 ut june 28 it exhibited left lateral strike slip faulting on a northeast striking and steeply dipping plane the big bear aftershocks form a linear trend extending 20 km to the northeast with a scattered distribution to the north the landers mainshock occurred near the southernmost extent of the eastern california shear zone an 80 km wide more than 400 km long zone of deformation this zone extends into the death valley region and accommodates about 10 to 20 of the plate motion between the pacific and north american plates the joshua tree preshock its aftershocks and landers aftershocks form a previously missing link that connects the eastern california shear zone to the southern san andreas fault

shear zones formed along long straight traces of fault zones during the 28 june 1992 landers california earthquake surface rupturing during the 28 june 1992 landers california earthquake east of los angeles accommodated right lateral offsets up to about 6 m along segments of distinct en echelon fault zones with a total length of 80 km the offsets were accommodated generally not by faults distinct slip sur faces but rather by shear zones tabular bands of localized shearing along simple stretches of fault zones at landers the rupture is characterized by telescoping of shear zones and intensification of shearing broad shear zones of mild sheafing containing narrow shear zones of more intense sheafing containing even nar rower shear zones of very intense sheafing which may contain a fault thus the ground ruptured across broad belts of shearing with clearly defined subparallel walls oriented nw each broad belt consists of a broad zone of mild sheafing extending across its entire width 50 to 200 m and much nan ower a few meters wide shear zones that accommodate most of the offset of the belt and are por trayed by en echelon tension cracks in response to right lateral shearing the slices of ground bounded by the tension cracks rotated in a clockwise sense producing left lateral sheafing and the slices were forced against the walls of the shear zone producing thrusting even narrower shear zones formed within the narrow shear zones although these probably are guides to right lateral fault segments below the surface rupturing during the earthquake is characterized not by faulting but by the formation of shear zones at various scales

poroelastic rebound along the landers 1992 earthquake surface rupture maps of surface displacement following the 1992 landers california earthquake generated by interferometric processing of ers 1 synthetic aperture radar sar images reveal effects of various postseismic deformation processes along the 1992 surface rupture the large scale pattern of the postseismic displacement field includes large lobes mostly visible on the west side of the fault comparable in shape with the lobes observed in the coseismic displacement field this pattern and the steep displacement gradient observed near the emerson camp rock fault cannot be simply explained by afterslip on deep sections of the 1992 rupture models show that horizontal slip occurring on a buried dislocation in a poisson s material produces a characteristic quadripole pattern in the surface displacement field with several centimeters of vertical motion at distances of 10 20 km from the fault yet this pattern is not observed in the postseismic interferograms as previously proposed to explain local strain in the fault step overs peltzer et al 1996b we argue that poroelastic rebound caused by pore fluid flow may also occur over greater distances from the fault compensating the vertical ground shift produced by fault afterslip such a rebound is explained by the gradual change of the crustal rocks poisson s ratio value from undrained coseismic to drained postseismic conditions as pore pressure gradients produced by the earthquake dissipate using the poisson s ratio values of 0 27 and 0 31 for the drained and undrained crustal rocks respectively elastic dislocation models show that the combined contributions of afterslip on deep sections of the fault and poroelastic rebound can account for the range change observed in the sar data and the horizontal displacement measured at global positioning system gps sites along a 60 km long transect across the emerson fault savage and svarc 1997 using a detailed surface slip distribution on the homestead valley kickapoo and johnson valley faults we modeled the poroelastic rebound in the homestead valley pull apart a poisson s ratio value of 0 35 for the undrained gouge rocks in the fault zone is required to account for the observed surface uplift in the 3 5 years following the earthquake this large value implies a seismic velocity ratio vp vs of 2 1 consistent with the observed low vs values of fault zone guided waves at shallow depth li et al 1997 the sar data also reveal postseismic creep along shallow patches of the eureka peak and burnt mountain faults with a characteristic decay time of 0 8 years coseismic dilatant hardening locking process followed by post seismic pore pressure controlled fault creep provide a plausible mechanism to account for the decay time of the observed slip rate along this section of the fault

proterozoic mesozoic history of the central asian orogenic belt in the tajik and southwestern kyrgyz tian shan u pb 40ar 39ar and fission track geochronology and geochemistry of granitoids multimethod geochronology u pb zircon 40 ar 39 ar hornblende biotite feldspar apatite fission track on granitoids gneisses and cenozoic intramontane basin clastics of the gissar alai ranges south tian shan collisional belt west of the talas fergana fault elucidates a history of neoproterozoic magmatism late paleozoic magmatism and metamorphism and mesozoic cenozoic thermal reactivation zircon core and grain interior u pb ages of ca 2 7 2 4 2 2 1 7 1 1 0 85 and 0 85 0 74 ga tie the early evolution of the gissar alai ranges to that of the tarim craton at least part of the gissar range crystalline basement the garm massif shows u pb zircon crystallization ages of ca 661 552 ma median ca 609 ma again suggesting a tarim craton connection tarim collided with the middle tian shan block at ca 310 305 ma completing the protracted formation of the south tian shan collisional belt the massive gissar range granitoids intruded later ca 305 270 ma contemporaneous with peak barrovian type metamorphism in the garm massif rocks major and trace element compositions suggest that the gissar granitoid melts have continental arc affinity zircon e hf and whole rock e nd values of 2 1 to 6 9 and 2 7 to 7 2 respectively and hf isotope crustal model and nd isotope depleted mantle model ages of ca 1 0 1 2 and ca 1 1 2 2 ga respectively suggest significant input of precambrian crust in the gissar granitoid and garm orthogneiss melts consistent with the u pb ages of inherited and detrital zircons the distinct ca 661 552 ma garm gneiss crystallization ages and the ca 1 0 2 2 ga model ages and the lack of 2 4 3 4 ga model ages tie the garm gneisses and the reworked crust of the gissar range to the northern rim the kuqa and kolar sections of the tarim craton suggesting a united karakum tarim craton although about contemporaneous with widespread postcollisional magmatism in the entire tian shan the large volume and short duration of the gissar range magmatism including crustal thickening and prograde metamorphism during tarim craton middle tian shan block collision and formation and closure of an oceanic back arc basin the gissar basin indicate its origin in a distinct setting combined this likely resulted in midcrustal melting and upper crustal batholith emplacement mafic dikes and pipes intruded at ca 256 238 ma median ca 241 ma the source region of the parental melts was within the asthenospheric mantle the simplest interpretation for these basanites is that they were part of the tarim flood basalt province this would extend this province westward from the tarim craton into the southwestern tian shan and imply that the relatively short lived flood basalt event ca 290 270 ma was followed by much less voluminous but longer lasting hotspot magmatism the 40 ar 39 ar and detrital apatite fission track dates outline post gissar alai range granitoid emplacement cooling cimmerian collision events at the southern margin of asia late cretaceous crustal extension and local magmatism and early cenozoic shortening and burial in the far field of the india asia collision

slip tapers at the tips of faults and earthquake ruptures 1 slip gradients near the tips of earthquake ruptures and faults are typically linear for non interacting faults or earthquake ruptures tip tapers are scale invariant and about 1 2 orders of magnitude larger for faults than for earthquakes for fault tips interacting with other faults the taper can be as much as a factor of 10 greater than for non interacting faults for earthquake ruptures taper is also greater by as much as a factor of 10 for tips of rupture segments in the interior of the main rupture tips in which the earthquake has propagated to the end of the fault and tips which interact with the stress drop of a previous earthquake on the same fault tip taper for faults increases with the strength of the wall rock these observations are consistent with fracture mechanics models in which inelastic deformation occurs in a volume surrounding the crack tip

re os pyrite and u pb zircon geochronology from the taldybulak levoberezhny gold deposit insight for cambrian metallogeny of the kyrgyz northern tien shan abstract the taldybulak levoberezhny gold deposit located in the eastern part of the kyrgyz northern tien shan is hosted in highly deformed precambrian schist and gneisses that have undergone intense quartz carbonate fuchsite and tourmaline alterations gold mineralization is ultimately subdivided into two stages based on the observation of alteration assemblages orebody geometries and the occurrences of au bearing minerals negative thermal ionization mass spectrometry re os isotopic analyses of five au rich pyrite samples from the early stage yielded an isochron age of 511 18 ma zircon sensitive high resolution ion microprobe u pb dating of a diorite dike sample postdating the late stage mineralization yielded a wide range of ages from 3055 to 291 ma while a weighted mean 206 pb 238 u age of 414 6 6 8 ma is believed to represent the age of dike intrusion and the upper limit on the timing of the late stage quartz tourmaline gold formation the pyrite 187 os 188 os initial ratio of 0 132 0 011 together with os values varying from 0 to 14 indicate a major mantle component for the source of os and by inference ore metals which may be linked to the ophiolite suite of the kopurelisai complex in the taldybulak levoberezhny area considering the geodynamic setting of the kyrgyz northern tien shan during the early paleozoic we suggest that cambrian mineralization of the taldybulak levoberezhny deposit can be attributed to a subduction related setting probably associated with the earliest accretion of the northern tien shan

geology geochronology and s pb os geochemistry of the alastuo gold deposit west tianshan nw china the alastuo gold deposit located in the narati region of the chinese west tianshan is a newly discovered gold deposit within the world class tianshan gold belt the gold mineralization is hosted by an early carboniferous granodiorite intrusion and controlled structurally by subsidiary faults of the transcrustal north narati fault the deposit consists of auriferous alteration assemblages and minor vein ores field relations and petrographic data suggest four stages of mineralization with gold precipitated coevally with quartz chlorite epidote pyrite and galena during the second and third stages that are characterized by tectonic transformation from ductile shearing to brittle deformation the temperature range of gold precipitation is estimated to be 280 330 c using chlorite geothermometry ten auriferous pyrite samples yielded a well defined re os isochron age of 325 3 ma and two sericite samples yielded plateau like 40ar 39ar ages of 321 7 3 0 ma and 321 2 2 8 ma these consistent ages suggest that the gold mineralization in the narati region was emplaced in a transitional regime from subduction to continental collision between the tarim block and middle tianshan terrane 325 310 ma rather than during the post collisional stage as in most part of the tianshan gold belt gold bearing pyrite has an initial osi value of 0 7 0 1 and lead isotopes ratios of 17 897 to 18 723 for 206pb 204pb 15 474 to 15 643 for 207pb 204pb and 37 522 to 38 299 for 208pb 204pb while samples of pyrite galena and sphalerite have 34sv cdt of 1 9 to 7 8 the isotope data suggest that the sulfur and metals are most likely derived from mixed mantle and crustal sources including metamorphic devolatilization of subducted oceanic slab and overlying sedimentary rocks the mineralization styles structural controls metal and sulfur sources and the timing of gold mineralization with respect to the orogenic event all suggest that the alastuo gold deposit represents an orogenic type deposit this newly recognized carboniferous orogenic gold mineralization event calls for a re evaluation of the gold metallogeny in the middle tianshan

mesoproterozoic grenville age terranes in the kyrgyz north tianshan zircon ages and nd hf isotopic constraints on the origin and evolution of basement blocks in the southern central asian orogen abstract the north tianshan orogenic belt in kyrgyzstan consists predominantly of neoproterozoic to early paleozoic assemblages and tectonically interlayered older precambrian crystalline complexes and formed during early paleozoic accretionary and collisional events one of the oldest continental fragments of late mesoproterozoic grenvillian age occurs within the southern part of the kyrgyz north tianshan using shrimp zircon ages we document two magmatic events at 1 1 and 1 3 ga the younger event is characterized by voluminous granitoid magmatism between 1150 and 1050 ma and is associated with deformation and metamorphism the older event is documented by 1 3 ga felsic volcanism of uncertain tectonic significance and may reflect a rifting episode geochemical signatures as well as nd and hf isotopes of the mesoproterozoic granitoids indicate melting of still older continental crust with model ages of ca 1 2 to 2 4 ga the mesoproterozoic assemblages are intruded by paleozoic diorites and granitoids and nd and hf isotopic systematics suggest that the diorites are derived from melts that are mixtures of the above mesoproterozoic basement and mantle derived material their source is thus distinct from that of the mesoproterozoic rocks emplacement of these plutons into the precambrian rocks occurred between 461 and 441 ma this is much younger than previously assumed and indicates that small plutons and large batholiths in north tianshan were emplaced virtually synchronously in the late ordovician to early silurian the mesoproterozoic rocks in the north tianshan may be remnants of a once larger continental domain whose fragments are preserved in adjacent blocks of the central asian orogenic belt comparison with broadly coeval terranes in the kokchetav area of northern kazakhstan the chinese central tianshan and the tarim craton point to some similarities and suggests that these may represent fragments of a single mesoproterozoic continent characterized by a major orogenic event at 1 1 ga known as the tarimian orogeny

the cambrian initiation of intra oceanic subduction in the southern paleo asian ocean further evidence from the barleik subduction related metamorphic complex in the west junggar region nw china abstract in this study we present new evidence from the barleik subduction related metamorphic complex in the southern west junggar region northwestern china for the cambrian initiation of intra oceanic subduction in the southern paleo asian ocean the barleik metamorphic complex is mainly composed of blueschist and amphibolite blocks within an ophiolitic melange and their protoliths are calc alkaline andesite and alkali and tholeiitic basalts the calc alkaline andesite has a zircon u pb age of 502 2 ma obtained from magmatic cores of zircon grains and shares geochemical features similar to the 515 485 ma intra oceanic arc magmatic rocks in the west junggar region by contrast the alkali and tholeiitic basalts have trace element features similar to ocean island and enriched mid ocean ridge basalts respectively rutile and sodic calcic amphibole from the amphibolite have a u pb age of 502 25 ma and a 40 ar 39 ar age of 504 ma respectively which are in good agreement within errors with a 40 ar 39 ar age of 492 4 ma for phengite from the blueschist these metamorphic ages of 500 ma are interpreted to represent the timing of pacific type subduction related metamorphism and are also compatible with ages of the oldest supra subduction zone ophiolites 531 512 ma and intra oceanic arc plutons 515 485 ma in the southern west junggar region being one of the oldest subduction related metamorphic complexes 509 490 ma in the southern central asian orogenic belt the barleik metamorphic complex together with the oldest arc plutons definitely indicate the initial intra oceanic subduction in the southern paleo asian ocean at least in the early cambrian

thermo tectonic history of the issyk kul basement kyrgyz northern tien shan central asia abstract lake issyk kul occupies a large late mesozoic cenozoic intramontane basin between the mountain ranges of the northern kyrgyz tien shan these ranges are often composed of granitoid basement that forms part of a complex mosaic assemblage of microcontinents and volcanic arcs several granites from the terskey kungey trans ili and zhetyzhol ranges were dated with the zircon u pb method shrimp la icp ms and yield concordant late ordovician silurian 456 420 ma emplacement ages these constrain the caledonian accretion history of the northern kyrgyz tien shan in the amalgamated palaeo kazakhstan continent the ancestral tien shan orogen assembled in the early permian when final closure of the turkestan ocean ensued collision of palaeo kazakhstan and tarim a late palaeozoic structural basement fabric formed and middle late permian post collisional magmatism added to crustal growth of the tien shan permo triassic cooling 300 220 ma of the ancestral tien shan was unraveled using 40 ar 39 ar k feldspar and titanite fission track ft thermochronology on the issyk kul granitoids apatite thermochronology ft and u th sm he applied to the broader issyk kul region elucidates the meso cenozoic thermo tectonic evolution and constrains several tectonic reactivation episodes in the jurassic cretaceous and cenozoic exhumation of the studied units occurred during a protracted period of intracontinental orogenesis linked to far field effects of late jurassic cretaceous accretion of peri gondwanan blocks from the tethyan realm to eurasian following a subsequent period of stability and peneplanation incipient building of the modern tien shan orogen in northern kyrgyzstan started in the oligocene according to our data intense basement cooling in distinct reactivated and fault controlled sections of the trans ili and terskey ranges finally pinpoint important miocene pliocene 22 5 ma exhumation of the issyk kul basement late cenozoic formation of the tien shan is associated with ongoing indentation of india into eurasia and is a quintessential driving force for the reactivation of the entire central asian orogenic belt

fault branching and rupture directivity 1 could the directivity of a complex earthquake be inferred from the ruptured fault branches it created typically branches develop in forward orientation making acute angles relative to the propagation direction direct backward branching of the same style as the main rupture e g both right lateral is disallowed by the stress field at the rupture front here we propose another mechanism of backward branching in that mechanism rupture stops along one fault strand radiates stress to a neighboring strand nucleates there and develops bilaterally generating a backward branch such makes diagnosing directivity of a past earthquake difficult without detailed knowledge of the branching process as a field example in the landers 1992 earthquake rupture stopped at the northern end of the kickapoo fault jumped onto the homestead valley fault and developed bilaterally there nnw to continue the main rupture but also sse for 4 km forming a backward branch we develop theoretical principles underlying such rupture transitions partly from elastostatic stress analysis and then simulate the landers example numerically using a two dimensional elastodynamic boundary integral equation formulation incorporating slip weakening rupture this reproduces the proposed backward branching mechanism based on realistic if simplified fault geometries prestress orientation corresponding to the region standard lab friction values for peak strength and fracture energies characteristic of the landers event we also show that the seismic s ratio controls the jumpable distance and that curving of a fault toward its compressional side like locally along the southeastern homestead valley fault induces near tip increase of compressive normal stress that slows rupture propagation

formation of the kokchetav subduction collision zone northern kazakhstan insights from zircon u pb and lu hf isotope systematics abstract the kokchetav subduction collision zone is located in the western part of the ancestral central asian orogenic belt this zone is built up by the precambrian kokchetav microcontinent which includes a hp uhp metamorphic belt and the north kokchetav tectonic zone nktz which represents an accretionary complex between the kokchetav microcontinent and the adjacent stepnyak island arc the entire region is widely intruded by palaeozoic granitoids which were emplaced after the collision of the stepnyak island arc with the kokchetav microcontinent we present zircon u pb ages and lu hf systematics in zircon to better characterize the tectonic evolution of the kokchetav subduction collision zone the lu hf results indicate that the kokchetav basement rocks are derived from late neoarchaean early palaeoproterozoic 2 5 ga crust for the granite gneiss basement of the kokchetav microcontinent early mesoproterozoic grenville age 1 17 1 14 ga zircon u pb crystallization ages were obtained for the nktz two main age components were recognized 1 an oldest mesoproterozoic age component 1 20 1 05 ga similar as for the kokchetav microcontinental zircons and 2 a younger early cambrian 540 520 ma or late cambrian early ordovician 490 480 ma age component th u ratios 0 1 and are typical of a magmatic protolith the distinction between both zircon types is supported by characteristic cathodoluminescence images these results confirm previous observations indicating early palaeozoic high grade metamorphism 540 520 ma and collisional deformation 490 480 ma of the mesoproterozoic kokchetav zone as a result of subduction accretion and collision of the stepnyak island arc to the kokchetav microcontinent for two additional samples of the balkashin granitic complex early devonian 415 395 ma zircon crystallization ages magmatic th u ratios were obtained the occurrence of a mesozoic xenocryst within these leucogranites may indicate that they were emplaced in a continental arc setting

the correlation of magnetic anisotropy with strain in the chelmsford formation of the sudbury basin ontario abstract a quantitative correlation has been established between finite strain and the anisotropy of magnetic susceptibility ams in sandstones of the precambrian chelmsford formation of the sudbury basin ontario investigations of the progressive acquisition of isothermal remanent magnetization and its destruction with reverse field and thermal demagnetization as well as microscopic examination of polished thin sections show that both pyrrhotite and magnetite are present in the sandstone measurements of the ams have been carried out at 16 sites distributed throughout the chelmsford formation outcrop considerable variation in the shapes of the magnetic susceptibility ellipsoids axes k max k int k min exists within each site with the k max and k int axes contained in the cleavage planes where developed the magnitudes of the magnetic susceptibility axes have been quantitatively correlated with strain measurements carried out by clendenen et al this issue two different strain to magnetics correlations have been carried out between the site mean strain determined using deformed concretions and the mean values of the ams at each site and between individual specimen strain values determined at the microscopic scale and the individual specimen ams in both cases separate correlations were made between strain long intermediate and short axes and their corresponding magnetic k max k int and k min axes at 10 sites the site mean strain values were derived from strain analysis of the concretions and at an additional 6 sites the site mean strain values were derived from the ams to strain correlation 527 individual cores were converted to equivalent strain values using the ams structural correlation of individual specimens this study extends the use of ams methods to include strain determination in sandstones and provides an order of magnitude increase in the available number of such data for further structural analysis

nonuniform prestress from prior earthquakes and the effect on dynamics of branched fault systems 1 to examine the effects of branched fault geometry on the dynamics of fault systems in the long term we perform multicycle simulations on generic faulting models an explicit finite element algorithm is used to simulate spontaneous dynamic rupture of earthquakes the fault stress during the interseismic period is evaluated by an analytical viscoelastic model we find that the fault prestress field becomes highly nonuniform near the branch point and on the two branch segments over multiple earthquake cycles owing to the branched fault geometry and stress interaction between the two segments the principal prestress on faults rotates over multiple earthquake cycles and departs from the regional stress field significantly near the branch point after a number of earthquake cycles the branched fault systems evolve to a steady state in which several patterns of the fault prestress and earthquake rupture repeat the nonuniform prestress developed from previous earthquakes has large effects on the rupture and slip patterns several different rupture scenarios can occur on a given branched fault system in addition backward branching can occur in the nonuniform prestress field either driven by slip on the stem of the fault system or through a triggering mechanism these modeling results may have important implications for understanding fault branching behavior observed in the 1992 landers the 1999 hector mine and the 2002 denali fault earthquakes and for seismic hazard analysis in the areas where branched fault systems exist

dynamic shear rupture interactions with fault bends and off axis secondary faulting 1 on the basis of elastodynamic stress fields for singular crack and nonsingular slip weakening models of propagating rupture we develop preliminary answers to such questions as follows if a rupturing fault is intersected by another providing a possible bend in the failure path when will stressing be consistent with rupture along the bend what secondary fault locations and orientations in a damaged region bordering a major fault will be stressed to failure by the main rupture stresses that could initiate rupture on a bend are shown to increase dramatically with crack speed especially near the limiting speed rayleigh for mode ii shear for mode iii whether a bend path once begun can be continued to larger scales depends on principal stress directions and ratios in the prestress field conditions should often be met in mode ii for which bend paths encouraged by stressing very near the rupture tip are discouraged by the larger scale stressing a basis for intermittent rupture propagation and spontaneous arrest secondary failure in the damage zone likewise increases markedly as the limiting speed is approached such may make the fracture energy much greater than for slip on a single surface the extent of secondary faulting is strongly affected by prestress directions and the ratio of residual to peak strength for mode ii prestress controls whether activation occurs primarily on the extensional side which we show to be the typical case or on the compressional side too natural examples are consistent with the concepts developed

zircon u pb and muscovite 40ar 39ar geochronology of the gold bearing tianger mylonitized granite xinjiang northwest china implications for radiometric dating of mylonitized magmatic rocks abstract this study focuses on zircon u pb and muscovite 40ar 39ar geochronology of gold bearing mylonitized granite in the north tianshan of china all zircon samples have euhedral hydrothermal rims which have replaced their igneous mantles igneous zircon mantles with narrow hydrothermal rims in sample ts06 give a weighted average u pb age of 403 7 5 6 ma 1 standard error mswd 1 5 n 18 zircons in ts277 show a complex texture with core detrital origin 1276 2215 ma indicating igneous mantle and a hydrothermal rim the igneous mantles give a weighted average u pb age of 443 4 3 3 ma mswd 3 1 n 10 two younger ages 394 and 400 ma have been obtained on hydrothermal rims the mantles plus rims of all zircons have positive ehf t values with an average of 3 99 0 86 mswd 2 8 n 23 the positive ehf t values suggest that the granite magma is derived from melting of juvenile crustal rocks the shearing deformation post dated granite intrusion and introduced fluid which replaced igneous zircons via dissolution and reprecipitation however the age data for zircon hydrothermal rims do not represent the time of the dissolution re precipitation the zircons crystallized in granite in the early silurian 443 ma have been replaced on their rims during middle triassic deformation 248 238 ma which was followed by brittle deformation accompanied by gold deposition at 220 ma based on 40ar 39ar dating of muscovite

palaeozoic uplands and unconformity in the north china block constraints from zircon la icp ms dating and geochemical analysis of bauxite the problem of location of the north china block ncb during the palaeozoic and its history of uplift and erosion have not been established satisfactorily a regional unconformity was developed in the ncb between the upper ordovician and the middle upper carboniferous zircon grains in the samples collected for isotopic age dating from the unconformable surface in central north china show two main clusters of ages around c 444 and c 313 ma and a scatter of ages between c 2500 and c 550 ma silurian and devonian ages were not observed similarities between the zircon rare earth element and trace elements show that the c 444 and c 313 ma zircon grains were derived from rocks in the active margins of the ncb geological and isotopic evidence suggests that between c 444 and 312 ma north china was uplifted and that there was a long gap in sedimentation this major change in the environment of north china from marine to continental upland occurred at the same time as the separation of north china from northern gondwana the closure of the northern palaeotethys along the altai inner mongolia zone and the collision between gondwana and laurasia

kinematic interpretation of the 1992 landers earthquake abstract the m w 7 3 landers earthquake produced approximately 80 km of surface faulting along a preexisting integrated system of faults separated by complex intersection points the earthquake was accompanied by distributed seismicity and triggered slip across a large area of the central mojave desert known as the mojave block we synthesized surface mapping geodetic and seismicity data to develop a kinematic model for regional co seismic deformation we interpret that the earthquake simultaneously accommodated north directed motion of blocks within the san bernardino mountains dextral shear on northwest striking faults in the central mojave block and minor northwest southeast extension of the eastern mojave block this pattern of distributed heterogeneous co seismic deformation is similar to a kinematic model proposed for cumulative late cenozoic deformation of the mojave block in which dextral shear north south shortening and northwest southeast extension are partitioned into discrete domains bounded principally by strike slip faults based on the similarity between the co seismic deformation and patterns of cumulative late cenozoic deformation we propose that infrequent multiple fault segment ruptures similar to those associated with the 1992 landers earthquake are part of the process by which the mojave block has changed size and shape during the late cenozoic this model suggests that combinations of multiple fault or multiple segment rupture may be assessed as relatively more or less likely depending on whether a particular rupture scenario contributes proportionally to the cumulative regional deformation

multi stage gold mineralization in the taldybulak levoberezhny deposit tien shan kyrgyzstan reply to comment by boris trifonov on re os pyrite and u pb zircon geochronology from the taldybulak levoberezhny gold deposit insight for cambrian metallogeny of the kyrgyz northern tien shan abstract our recent geochronological study on the taldybulak levoberezhny gold deposit in kyrgyz northern tien shan knts received critical comments by boris trifonov 2016 regarding the age of dikes and its relationship with gold mineralization at the deposit in this contribution based on a re evaluation of our published data together with new geochronological geochemical and sr nd hf isotopic data the age of gold mineralization magmatic and tectonic events and the relationship between dikes and mineralization at the deposit it is proposed that the taldybulak levoberezhny gold deposit is a product of long lived magmatic hydrothermal system with multiple stages of gold mineralization from cambrian to late carboniferous early permian

estimating prehistoric earthquake magnitude from point measurements of surface rupture abstract we have developed a method for estimating the magnitude of prehistoric earthquakes using displacement data that usually can be collected from paleoseismic investigations this method is necessary because essentially all current magnitude estimates for prehistoric events rely on determining the total length of coseismic surface rupture which is rarely measurable or rely on segmentation scenarios for which uncertainties cannot be quantified although surface rupture length is a better predictor of magnitude than displacement for historic earthquakes paleoseismic investigations are better at providing measurements of the amount of displacement at a site along a fault the key to our method is to incorporate the variability in displacement observed in 14 modern events which allows a formal uncertainty in magnitude to be assigned to prehistoric ruptures we show how multiple measurements along a preserved fraction of a rupture can be combined to reduce the uncertainty in the estimate of magnitude our analysis shows that uncertainty asymptotically approaches the natural variability of ruptures so 5 to 10 displacement measurements are sufficient to characterize paleomagnitude we conclude that sampling of scarps with lengths of even 10 of the original rupture can provide magnitude values that reasonably estimate the earthquake tests of the method using randomly sampled data from the 1992 m w 7 3 landers and 1954 m s 6 8 dixie valley earthquakes provide close approximations of the actual magnitudes

structural characterization of the katebasu gold deposit xinjiang china tectonic correlation with the amalgamation of the western tianshan abstract the western tianshan orogen is one of the world s most important gold provinces it hosts numerous world class gold deposits along the southern and northern margins of the middle tianshan most deposits occur within large scale fault zones and or ductile brittle deformation zones that were formed during the paleozoic evolution of the paleo asian ocean thereby indicating a correlation between orogenic tectonics and the gold mineralization however the correlation is often poorly constrained and remains controversially discussed the katebasu gold deposit in xinjiang china is located on the northern margin of middle tianshan and is hosted by secondary fault zones to the south of the north nalati fault the main period of gold mineralization is suggested to be the late carboniferous which is broadly coeval with the late paleozoic amalgamation of the western tianshan therefore the katebasu gold deposit provides an excellent opportunity to search for possible correlations between structural processes at a deposit scale and orogenic processes at a regional scale to better understand the tectonic controls on large scale gold mineralization in the western tianshan in this paper we report detailed structural mapping and field investigations on the fault geometries and kinematics of the katebasu gold deposit that permit inversion of the tectonic stress field our structural data combined with results of previous studies of the regional tectonics allow us to reconstruct the structural history of the katebasu gold deposit to look for possible correlations with the orogenic processes in the western tianshan and to determine their control on gold mineralization the first stage of deformation d1 was top to the north thrusting that was caused by a roughly n s compression and is represented by ene wsw striking thrust faults d1 was likely related to the closure of the south tianshan ocean and the resulting collision between the tarim craton and middle tianshan yili block the second stage of deformation d2 was dextral strike slip motion along the ene wsw striking faults that was caused by the wnw ese stress field and that corresponds to permian dextral wrenching in the tianshan orogen subsequent ne sw compression led to the third deformation event d3 that mainly formed nw se striking reverse faults and nne ssw striking dextral faults

influence of releasing step overs on surface fault rupture and fault segmentation examples from the 17 august 1999 izmit earthquake on the north anatolian fault turkey the 17 august 1999 izmit earthquake broke four distinct structural segments of the north anatolian fault from east to west the karadere sakarya sapanca and golcuk fault segments these segments are separated from one another and the adjoining yalova and duzce fault segments to the west and east respectively by distinct right releasing step over and or gaps in the fault trace these include from east to west the eften lake step over 2 4 km wide akyazi gap 6 13 km long sapanca step over 1 2 km wide golcuk step over 1 2 km wide and karamursel step over 4 5 km wide the earthquake nucleated within or near the golcuk step over with bilateral rupture propagation to the west and east to the west about 4 5 5 m of surface rupture up to 7 m of subsurface rupture on the golcuk segment was mostly if not entirely arrested by the 4 to 5 km wide karamursel step over to the east about 3 4 m of surface rupture about 4 m of subsurface rupture propagated through the 1 to 2 km wide sapanca step over triggering up to 4 5 5 5 m of surface rupture up to 5 6 m of subsurface rupture on the sakarya segment rupture on the sakarya segment stepped across the akyazi gap triggering 1 1 5 m of rupture on the karadere segment rupture on the karadere segment was arrested by the 2 to 4 km wide eften lake step over modeling of the transient stress field associated with the rupture propagation agrees well with the field observations the modeled fault rupture of 5 m on the golcuk segment appears to decrease the transient stress on the eastern tip of the adjoining yalova segment across the 4 to 5 km wide karamursel step over even if we allow rupture on the golcuk segment to extend along the entire southern margin of the step over our model shows that regardless of the amount of slip in a generic sense it is difficult to increase stress on the tip of an adjoining fault segment across a 5 km wide step over on the other hand fault rupture of 3 m on the western tip of the sapanca fault segment significantly increases the transient stress on the adjoining sakarya segment across the 1 to 2 km wide sapanca step over allowing rupture propagation across the basin an evaluation of 30 historical strike slip earthquakes involving 59 step over basins supports our field observations and stress modeling of the izmit fault rupture historical strike slip events with small to large displacements usually propagate through step overs less than 1 2 km wide with increasing displacement larger and larger step overs can be ruptured through the empirical data generally show a ratio ranging from 2 1 to 1 1 between step over width in kilometers and strike slip displacement in meters for a through going rupture however step overs of 4 to 5 km width appear to always arrest fault rupture regardless of the amount of displacement these results therefore provide important constraints for evaluating the probability of multisegment ruptures on segmented strike slip faults manuscript received 20 august 2000

the 1999 izmit turkey earthquake a 3d dynamic stress transfer model of intraearthquake triggering before the august 1999 izmit kocaeli turkey earthquake theoretical studies of earthquake ruptures and geological observations had provided estimates of how far an earthquake might jump to get to a neighboring fault both numerical simulations and geological observations suggested that 5 km might be the upper limit if there were no transfer faults the izmit earthquake appears to have followed these expectations it did not jump across any step over wider than 5 km and was instead stopped by a narrower step over at its eastern end and possibly by a stress shadow caused by a historic large earthquake at its western end our 3d spontaneous rupture simulations of the 1999 izmit earthquake provide two new insights 1 the west to east striking fault segments of this part of the north anatolian fault are oriented so as to be low stress faults and 2 the easternmost segment involved in the august 1999 rupture may be dipping an interesting feature of the izmit earthquake is that a 5 km long gap in surface rupture and an adjacent 25 restraining bend in the fault zone did not stop the earthquake the latter observation is a warning that significant fault bends in strike slip faults may not arrest future earthquakes manuscript received 30 august 2000