late cretaceous extensional unroofing in the funeral mountains metamorphic core complex california new field and geochronologic data document the existence of late cretaceous extensional structures in the death valley region california nevada we have mapped two major low angle ductile shear zones that omit stratigraphy in the footwall of the funeral mountains metamorphic core complex intervening strata have been strongly attenuated although stratigraphic offset across the shear zones is only 1 5 km the presence of a large metamorphic discontinuity suggests that the amount of unroofing must be much greater the timing of shear zone formation attenuation and subsequent northwest vergent folding is constrained by u pb geochronology on 1 prekinematic or synkinematic and 2 postkinematic pegmatites deformation was taking place by 72 ma and had ended by 70 ma these results support earlier petrologic and geochronologic data that suggested substantial unroofing of the funeral mountains in late cretaceous time and add to a growing body of evidence for widespread mesozoic extension in the hinterland of the sevier thrust belt

role of crustal flexure in initiation of low angle normal faults and implications for structural evolution of the basin and range province the mechanics of low angle normal faulting has proved difficult to establish despite clear geological evidence for the existence of such faults gently dipping shear stress trajectories are necessary for nucleation and propagation of low angle faults in a structurally isotropic medium such trajectories are not produced by simple horizontal extension but do arise in the presence of flexural stresses we model stress conditions in a cross section through the strong upper crust using airy stress functions for a two dimensional elastic medium numerical modeling indicates that high ratios of flexural stress to extensional stress produce conditions favorable for low angle normal fault initiation at intermediate depths in an elastic medium we propose that the gentle dip of cenozoic low angle normal faults in the basin and range province resulted from flexural stresses that were produced by isostatically uncompensated surface and especially moho relief resurgent mid cenozoic magmatism and the consequent reduction of the thermally dependent flexural strength of the mantle lithosphere transferred flexural stresses to the upper crust causing or promoting initiation of low angle normal faults temporal changes in structural style from dominantly low angle to dominantly high angle normal faulting may reflect reduction of the magnitude of flexural stresses in the upper crust due to flattening of the moho and cooling of the mantle lithosphere

structural petrological and thermal evolution of a tertiary ductile strike slip shear zone diancang shan yunnan the diancang shan a horst massif within the red river fault zone in yunnan people s republic of china preserves a structural petrological and thermal record of two distinct phases of tectonic activity a left lateral ductile shear that terminated between 20 and 17 ma and a ductile to brittle phase of normal faulting which began at 4 7 ma and remains active mylonitic rocks in the core of the range display an early steep high temperature ht schistosity and a horizontal stretching lineation that are both parallel to the trend of the belt kinematic indicators indicate that shear was left lateral the complex shape of the ht schistosity at the southern termination of the massif likely results from a large scale oblique left lateral c shear plane that dismembered the shear zone and separated the ailao shan and the diancang shan as left lateral deformation terminated thermochronological and thermobarometric results suggest that the gneisses were partially unroofed during this event along the eastern edge of the diancang shan the ht fabrics were overprinted by low temperature structures during activation of east dipping normal faults cooling associated with this normal right lateral faulting along the diancang shan and perhaps activation of the right lateral normal movement on the range front fault farther south along the ailao shan began at 4 7 0 1 ma these results tend to support the view that extrusion of indochina occurred along the left lateral red river shear zone between 35 and 19 17 ma initiation of right lateral normal slip during the late miocene may relate to eastward extensional collapse of the thickened tibetan crust or more probably to initiation of the second phase of extrusion

simulated midlatitude summer nighttime anomaly in realistic geomagnetic fields 1 in the present work using the three dimensional theoretical ionospheric model of the earth in institute of geology and geophysics chinese academy of sciences time3d iggcas we simulate the temporal spatial distribution of the midlatitude summer nighttime anomaly msna and the influences of thermospheric meridional and zonal winds on the formation of msna msna mainly appears in three distinct regions the east asian region the northern atlantic europe region and the south pacific region msna in the south pacific region is obviously stronger than that in the other two regions in these three regions msna not only occurs in local summer but also often occurs in equinox through lifting up or lowering the f region ionization thermospheric winds mainly drive the formation of msna in the east asian region and the northern atlantic europe region msna are mainly driven by the thermospheric meridional wind and the influence of the thermospheric zonal wind can be neglected however although msna in the south pacific region or weddell sea anomaly are mainly driven by the thermospheric meridional wind the thermospheric zonal wind also plays an important role in the formation of msna

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

isostatic rebound due to tectonic denudation a viscous flow model of a layered lithosphere a four layer model of the upper 150 km of the earth is used to calculate the viscous response of continental crust and the underlying mantle to tectonic denudation the model comprises a strong upper crustal layer a weak lower crustal layer a very strong mantle lithosphere layer and a weak mantle asthenosphere layer which is in accord with experimental constraints on strength depth profiles for continental lithosphere the strength of each layer is represented by its effective viscosity flow in the crust and mantle is driven by buoyancy forces which arise from the unloading of an allochthon along a detachment fault by a series of instantaneous displacements earthquakes or rapid creep events numerical solutions obtained by using a finite element technique predict footwall uplift moho deflection and surface topography that are consistent with observations from the basin and range province of the western united states the calculated curvature of the footwall uplift is also similar to that observed and is sensitive to the geometry of the detachment fault such bending need not be elastically controlled hence the curvatures of footwall domes do not clearly place limits on the effective elastic thickness of the extending crust the upward deflection of the moho and the surface topography are sensitive to the viscosity structure and enable us to bound the range of the various viscosities by matching observations from the basin and range province which indicate no moho deflection and low magnitude of surface topography 3 5 km we estimate the upper crustal lower crustal and mantle lithospheric viscosities in the ranges 1021 1023 pa s 1019 1021 pa s and 1021 1023 pa s respectively

timing of detachment faulting in the bullfrog hills and bare mountain area southwest nevada inferences from 40ar 39ar k ar u pb and fission track thermochronology crustal extension in the bullfrog hills and bare mountain area of southwest nevada is associated with movement along a regional detachment fault normal faulting in the upper plate and rapid cooling denudation of the lower plate were coeval with miocene silicic volcanism and with west northwest transport along the detachment fault a west northwest progression of tilting along upper plate normal faults is indicated by ages of the volcanic rocks in relation to angular unconformities near the breakaway tilting in the upper plate occurred between 12 7 and 11 6 ma continued less strongly past 10 7 ma and was over by 8 2 ma ten to 20 km west of the breakaway tilting occurred between 10 7 and 10 33 ma continued less strongly after 10 33 ma and was over by 8 1 ma the cooling histories of the lower plate metamorphic rocks were determined by thermochronologic dating methods k ar and ar 40 39 a on muscovite biotite and hornblende ar 40 39 a on k feldspar u pb on apatite zircon and sphene and fission track on apatite zircon and sphene lower plate rocks 10 km west of the breakaway cooled slowly from early cretaceous lower amphibolite facies conditions through 350 50 degrees to 300 50 degrees c between 57 and 38 ma then cooled rapidly from 205 50 degrees to 120 5o degrees c between 12 6 1 6 and 11 1 1 9 ma lower plate rocks 20 km west of the breakaway cooled slowly from early cretaceous upper amphibolite facies conditions through 500 50 degrees c at 78 67 ma passed through 350 50 degrees to 300 50 degrees c between 16 3 0 4 and 10 5 0 3 ma then cooled rapidly from 285 50 degrees to 120 50 degrees c between 10 2 and 8 6 ma upper plate tilting and rapid cooling denudation of the lower plate occurred simultaneously in the respective areas the early slow cooling part of the lower plate thermal histories was probably related to erosion at the earth s surface which stripped off about 9 km of material in 50 to 100 m y the results indicate an initial fault dip greater than or equal to 30 degrees and a 12 mm yr 1 west northwest migration of the locus of rapid tilting in the upper plate

40ar39ar analysis of perthite microtextures and fluid inclusions in alkali feldspars from the klokken syenite south greenland laser probe andin vacuo crushing have been used to apply the40ar39ar technique to microtexturally well characterised pristine and turbid alkali feldspar from the 1166 ma klokken syenite individual crystals are several millimetres in size but sem and tem studies reveal complex microstructures which define a range of sub grain sizes over three orders of magnitude from 100 m in pristine areas with fine scale cryptoperthite down to 200 nm in turbid areas with coarsened perthites and numerous micropores laser probe40ar39ar measurements of turbid feldspar gave low apparent ages indicating significant40ar loss 40 average which can be accounted for by sustained heating of the small sub grains 1 m for 1166 ma at low temperature 150 c pristine feldspar gives high apparent ages indicating the presence of excess40ar this component was readily lost from the feldspar by laboratory heating at low temperature 600 c and was also released by crushing which indicates that it was released from fluid inclusions during crushing release of excess40ar is correlated with cl release and the40ar cl ratio of the fluid is similar to that of mantle fluids turbid feldspar also released a fluid during crushing but the40ar cl ratio was lower probably as a result of evolution of the original fluid during boiling anomalously high apparent ages are often a feature of low temperature ar release in stepped heating age spectra of feldspars the presence of excess40ar in fluid inclusions may substantially affect the lower temperature release in feldspar stepped heating spectra the character of the microtextures in the host feldspar provides information on the temperature of fluid trapping because inclusions formed above the coherent alkali feldspar solvus have little or no effect on the exsolution textures whereas fluids trapped at low temperatures are associated with major structural rearrangements leading to development of patch perthite

argon loss by alkali feldspars abstract ar loss is generally thought to preclude the use of alkali feldspars for dating old rocks by the k ar method and has been thought to occur via structural features such as the boundaries of perthite lamellae or twins we have done a 40 ar 39 ar step heating study of a suite of alkali feldspars from the klokken syenite intrusion u pb age 1166 4 ma which have been fully characterized by transmission electron microscopy all are low albite low microcline intergrowths but some are non turbid strain controlled cryptoperthites while others are turbid coarse microperthites geological and textural evidence suggests that the turbidity developed by fluid rock interactions during the initial cooling of the pluton in the stability field of low microcline 5 a after crystallization the finest cryptoperthites have numerous fully coherent perthite lamellar boundaries lamellar periodicity 40 nm and twin composition planes the have retained radiogenic 40 ar and yield total degassing ages as high as 1125 16 ma and maximum ages on the age spectra of 1162 16 ma the coarse turbid microperthites with patchy intergrowths and twins on the mm scale gave total degassing ages as low as 650 ma maximum age 750 ma argon leakage occurs least in feldspars with the most lamellar interfaces but in proportion to the degree of turbidity which is thought to be caused by large numbers of micropores strain controlled intergrowths in perthites allow the recognition of volumes of alkali feldspar crystals which have not interacted with deuteric fluids these retain argon and can give reliable k ar ages in old rocks saddle shaped age spectra appear to result from the preferential degassing at 1000 c of part of the feldspar structure which has lost more of its argon in nature the laboratory degassing does not occur in a manner analogous to that which has occurred naturally

experimental study of thermal field evolution in the short impending stage before earthquakes phenomena at critical points are vital for identifying the short impending stage prior to earthquakes the peak stress is a critical point when stress is converted from predominantly accumulation to predominantly release we call the duration between the peak stress and instability the meta instability stage which refers to the short impending stage of earthquakes the meta instability stage consists of a steady releasing quasi static stage and an accelerated releasing quasi dynamic stage the turning point of the above two stages is the remaining critical point to identify the two critical points in the field it is necessary to study the characteristic phenomena of various physical fields in the meta instability stage in the laboratory and the strain and displacement variations were studied considering that stress and relative displacement can be detected by thermal variations and peculiarities in the full field observations we employed a cooled thermal infrared imaging system to record thermal variations in the meta instability stage of stick slip events generated along a simulated precut planer strike slip fault in a granodiorite block on a horizontally bilateral servo controlled press machine the experimental results demonstrate the following 1 a large area of decreasing temperatures in wall rocks and increasing temperatures in sporadic sections of the fault indicate entrance into the meta instability stage 2 the rapid expansion of regions of increasing temperatures on the fault and the enhancement of temperature increase amplitude correspond to the turning point from the quasi static stage to the quasi dynamic stage our results reveal thermal indicators for the critical points prior to earthquakes that provide clues for identifying the short impending stage of earthquakes

an early miocene transition in deformation regime within the red river fault zone yunnan and its significance for indo asian tectonics recent work has tended to support the view that lateral extrusion of indochina along the red river fault zone was a significant factor in accommodating the convergence of india with asia to better understand the nature and timing of this feature 40ar 39 ar age spectrum analyses were undertaken on minerals from the ailao shan red river metamorphic belt in southern yunnan china interpreted using diffusion domain theory these results yield thermal histories in the temperature range 550 150 c discontinuities in cooling rates observed from these results suggest a transition in deformation style in the shear zone during the early miocene at least in the vicinity of our samples left lateral strike slip ductile deformation appears to have ceased by about 20 ma the mylonitic gneisses in the shear zone were subsequently obliquely unroofed probably due to a component of normal faulting along the eastern edge of the shear zone causing a brief locally 100 c m y cooling episode this transition may have resulted from a component of northeast southwest extension attributable to greater clockwise rotation of indochina relative to south china after 20 ma than that consistent with pure strike slip along the fault k ar ages on clay minerals from gouge within the normal faults bounding the ailao shan and diancang shan yield a range of ages between 20 and 185 ma but show a significant clustering at 20 25 ma these results suggest that early miocene movement on a precursor to the range front normal fault in the ailao shan caused oblique unroofing of the shear zone this early miocene event corresponds in time with a transition from low to very high denudation rates throughout the himalaya and southern tibet this coincidence may reflect that once left lateral strike slip motion on the red river fault zone slowed or was terminated accommodation of much of the continued northward convergence of india occurred on the main central thrust resulting in rapid uplift in the tethyan himalaya and gangdese belt

anomalous diffuse co2 emission prior to the january 2002 short term unrest at san miguel volcano el salvador central america on january 16 2002 short term unrest occurred at san miguel volcano a gas and steamash plume rose a few hundred meters above the summit crater an anomalous microseismicity pattern about 75 events between 7 30 and 10 30 hours was also observed continuous monitoring of co2 efflux on the volcano started on november 24 2001 in the attempt to provide a multidisciplinary approach for its volcanic surveillance the background mean of the diffuse co2 emission is about 16 g m 2 d 1 but a 17 fold increase up to 270 g m 2 d 1 was detected on january 7 nine days before the january 2002 short term unrest at san miguel volcano these observed anomalous changes on diffuse co2 degassing could be related to either a sharp increase of co2 pressure within the volcanic hydrothermal system or degassing from an uprising fresh gas rich magma within the shallow plumbing system of the volcano since meteorological fluctuations cannot explain this observed increase of diffuse co2 emission

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

background stress state before the 2008 wenchuan earthquake and the dynamics of the longmen shan thrust belt a stress reconstruction was performed based on focal mechanisms around the longmen shan region prior to the 2008 ms 8 0 wenchuan earthquake using a newly developed algorithm known as mca the method determines the stress tensor including principal axes orientations and quantitative stress values such as the effective confining pressure and maximum shear stress the results of the mca application using data recorded by the regional network from 1989 to april 2008 show the background stress state around the longmen shan belt before the wenchuan earthquake the characteristics of the stress orientation reveal that the longmen shan region is primarily under the eastward extrusion of the eastern tibetan plateau non uniform quantitative stress distributions show low stress levels in the upper crust of the middle longmen shan segment which is consistent with the observed high angle reverse faulting associated with the 2008 wenchuan earthquake in contrast other study areas such as the bayankela block and the nw strip extending to the sichuan basin show high stress intensity this feature coincides with heterogeneity in the wave speed image of the upper crust in this region which shows high s wave speed in the high stress areas and comparatively low s wave speed in low stress areas deformation features across the longmen shan belt with the slow rates of convergence determined by gps and the distribution of surface deformation rates also are in keeping with our stress results we propose a dynamic model in which sloping uplift under the longmen shan which partly counteracts the pushing force from the eastern plateau causes the low quantitative stresses in the upper crust beneath the longmen shan the decreasing gravitational potential energy beneath the longmen shan leads to earthquake thrust faulting and plays an important role in the geodynamics of the area that results from ductile thickening of the deep crust behind the sichuan basin creating a narrow steep margin

stress state and fluid pressure level along the whipple detachment fault california brittle structures in the upper 200 m of the footwall of the whipple detachment fault are used to characterize the state of stress under which the detachment slipped they are younger than or coeval with chloride brecciation and those that record shear typically are either moderately northeast dipping normal structures or steep northwest striking southwest down structures that we interpret as conjugate shears from this we infer that the maximum principal stress was oriented 55 80 from the detachment while they were active the angle between the conjugate faults averages about 30 indicating failure in the transtensional regime during conjugate faulting the effective least principal stress was nearly equal to the tensile strength the whipple fault probably had a normal coefficient of static friction and it slipped under hydrostatic to only moderately elevated fluid pressure rapid mineralization in the surroundings of major fault zones allows transtensional failure there and permits differential stress to be higher than if cohesionless friction limits strength this causes resolved shear stress sufficient for slip on faults severely misoriented relative to the maximum principal stress

tectonically induced anomalies without large earthquake occurrences in this study we documented a case involving large scale macroscopic anomalies in the xichang area southwestern sichuan province china from may to june of 2002 after which no major earthquake occurred during our field survey in 2002 we found that the timing of the high frequency occurrence of groundwater anomalies was in good agreement with those of animal anomalies spatially the groundwater and animal anomalies were distributed along the anninghe zemuhe fault zone furthermore the groundwater level was elevated in the northwest part of the zemuhe fault and depressed in the southeast part of the zemuhe fault zone with a border somewhere between puge and ningnan counties combined with microscopic groundwater geodetic and seismic activity data we infer that the anomalies in the xichang area were the result of increasing tectonic activity in the sichuan yunnan block in addition groundwater data may be used as a good indicator of tectonic activity this case tells us that there is no direct relationship between an earthquake and these anomalies in most cases the vast majority of the anomalies including microscopic and macroscopic anomalies are caused by tectonic activity that is these anomalies could occur under the effects of tectonic activity but they do not necessarily relate to the occurrence of earthquakes

thermochronologic evidence of major tectonic denudation associated with detachment faulting northern ruby mountains east humboldt range nevada fission track ft thermochronology studies on igneous and metamorphic rocks from the ruby mountains east humboldt range metamorphic core complex provide important constraints on the timing and nature of major mid tertiary extension in northeast nevada rocks from within the ruby east humboldt detachment brittle ductile normal sense shear zone were analyzed the dominant lithology studied was variably mylonitic mafic orthogneiss nonmylonitic amphibolite from the top of the structurally lower migmatitic core and porphyritic biotite granodiorite from the oligocene circa 36 ma harrison pass pluton were also studied ft ages on apatite zircon and sphene except one 18 4 ma apatite from the basal portion of the detachment zone are concordant and range in age from 26 5 ma to 23 6 ma all ages overlap at 1 between 25 4 ma and 23 4 ma these data suggest that rocks of the upper and middle portions of the detachment zone cooled rapidly from temperatures above 285 c sphene closure temperature to below 70 c lower temperature limit of track stability in apatite near the beginning of the miocene minimum cooling rate of 40 c ma the length distribution of confined fission tracks in apatites from these exhumed middle crustal rocks is strikingly similar to those of rocks known to have cooled rapidly fish canyon ash flow tuff the lower part of the detachment zone as well as the underlying migmatitic core also cooled through sphene and zircon closure temperatures during this time interval but apparently only cooled to a temperature within the zone of partial annealing of apatite 70 130 c residence of these rocks in this zone led to a partial retention of fission tracks thus resulting in a reduced age confined track length distributions from apatite provide independent evidence of very rapid cooling but also indicate that the rocks of the lower detachment zone experienced a more protracted cooling history remained hotter longer than structurally higher levels of the zone ft data firmly establish the lower limit on the timing of mylonitization during detachment faulting in this area as 23 4 ma rapid cooling of the region is considered to reflect large scale tectonic denudation intracrustal thinning the vertical complement to crustal extension rocks originating in the middle crust 10 15 km were quickly brought near the surface along the ruby east humboldt detachment fault brittle ductile simple shear zone and juxtaposed against brittlely extended rocks deformed under upper crustal conditions

thermal and barometric constraints on the intrusive and unroofing history of the black mountains implications for timing initial dip and kinematics of detachment faulting in the death valley region california unroofing of the black mountains death valley california has resulted in the exposure of 1 7 ga crystalline basement late precambrian amphibolite facies metasedimentary rocks and a tertiary magmatic complex the 40ar 39ar cooling ages obtained from samples collected across the entire length of the range 55 km combined with geobarometric results from synextensional intrusions provide time depth constraints on the miocene intrusive history and extensional unroofing of the black mountains data from the southeastern black mountains and adjacent greenwater range suggest unroofing from shallow depths between 9 and 10 ma to the northwest in the crystalline core of the range biotite plateau ages from 13 to 6 8 ma from rocks making up the death valley turtlebacks indicate a midcrustal residence with temperatures 300 c prior to extensional unroofing biotite 40ar 39ar ages from both precambrian basement and tertiary plutons reveal a diachronous cooling pattern of decreasing ages toward the northwest subparallel to the regional extension direction diachronous cooling was accompanied by dike intrusion which also decreases in age toward the northwest the cooling age pattern and geobarometric constraints in crystalline rocks of the black mountains suggest denudation of 10 15 km along a northwest directed detachment system consistent with regional reconstructions of tertiary extension and with unroofing of a northwest deepening crustal section mica cooling ages that deviate from the northwest younging trend are consistent with northwestward transport of rocks initially at shallower crustal levels onto deeper levels along splays of the detachment the well known amargosa chaos and perhaps the badwater turtleback are examples of this splaying process considering the current distance of the structurally deepest samples away from moderately to steeply east tilted tertiary strata in the southeastern black mountains these data indicate an average initial dip of the detachment system of the order of 20 similar to that determined for detachment faults in west central arizona and southeastern california beginning with an initially listric geometry a pattern of footwall unroofing accompanied by dike intrusion progresses northwestward this pattern may be explained by a model where migration of footwall flexures occur below a scoop shaped hanging wall block one consequence of this model is that gently dipping ductile fabrics developed in the middle crust steepen in the upper crust during unloading this process resolves the low initial dips obtained here with mapping which suggests transport of the upper plate on moderately to steeply dipping surfaces in the middle and upper crust

modes of tilting during extensional core complex development crustal extension and formation of the mineral mountains core complex utah involved tilting of the mineral mountains batholith and associated faults during hanging wall and footwall deformation the batholith was folded in the hanging wall of the beaver valley fault between 11 and 9 million years ago yielding about 45 of tilt subsequently the batholith was unroofed along the cave canyon detachment fault and the batholith and fault were tilted approximately 40 during footwall uplift recognition of deformed dikes beneath the detachment fault establishes the importance of footwall tilt during formation of extensional core complexes and demonstrates that footwall rebound can be an important process during extension

anomalous streamflow and groundwater level changes before the 1999 m7 6 chi chi earthquake in taiwan possible mechanisms streamflow recorded by a stream gauge located 4 km from the epicenter of the 1999 m7 6 chi chi earthquake in central taiwan showed a large and rapid anomalous increase of 124 m3 s starting 4 days before the earthquake this increase was followed by a comparable co seismic drop to below the background level for 8 months in addition groundwater levels recorded at a well 1 5 km east of the seismogenic fault showed an anomalous rise 2 days before the earthquake and then a unique 4 cm drop beginning 3 h before the earthquake the anomalous streamflow increase is attributed to gravity driven groundwater discharge into the creek through the openings of existing fractures in the steep creek banks crossed by the upstream shueilikun fault zone as a result of pre earthquake crustal buckling the continued tectonic movement and buckling together with the downward flow of water in the crust may have triggered the occurrence of some shallow slow slip events in the shueilikun and other nearby fault zones when these events propagate down dip to decollement where the faults merges with the seismogenic chelungpu fault they may have triggered other slow slip events propagating toward the asperity at the hypocenter and the chelungpu fault these events may then have caused the observed groundwater level anomaly and helped to trigger the earthquake

tectonics of mid tertiary extension along a transect through west central arizona large magnitude miocene extension in west central arizona occurred primarily along three imbricate northeast dipping normal faults the structurally highest of these faults the gently dipping buckskin rawhide detachment fault accommodated approximately 66 km of crustal extension whereas the two structurally lower faults accommodated a total of about 20 km extension due to this large magnitude extension an area at the earth s surface that was 10 to 20 km wide is now over a 100 km wide and crystalline rocks with mid tertiary mylonitic fabrics uncovered by detachment faulting are exposed over roughly 2000 km in the harcuvar metamorphic core complex most of the upper plate of the buckskin rawhide detachment fault was largely undeformed by internal extension only the thin tapered end of the upper plate was highly extended during extension the lower plate must have flexed to conform to the listric underside of the upper plate and to have flattened to its present subhorizontal form as it was uncovered grooves on the underside of the upper plate were apparently imposed on the pliable lower plate as it was denuded forming extension parallel folds in the lower plate low flexural strength characterized the lower plate during denudation and a highly mobile low viscosity deeper crust must have effectively decoupled the upper crust from the mantle lithosphere