dextral shear along the eastern margin of the colorado plateau a kinematic link between laramide contraction and rio grande rifting ca 75 13 ma abstract kinematic data associated with both laramide age and style and rio grande rift related structures show that the latest cretaceous to neogene interaction between the colorado plateau and the north american craton was dominantly coupled with a component of dextral shear consistent with earlier studies minor fault data in this study yielded results of varied kinematics inverted to a common northeast oriented hemisphere the mean trend of kinematic shortening associated with laramide age structures is documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

paleomagnetism of the dewey lake formation late permian northwest texas end of the kiaman superchron in north america the remanent magnetization of red beds of the late permian dewey lake formation from northwest texas was investigated using thermal chemical and alternating field demagnetization after removal of a small present day viscous overprint the dewey lake shows univectorial decay of the remanence revealing a dual polarity characteristic direction of d 152 4 i 11 1 k 39 5 95 5 9 n 16 sites three polarity transitions were observed defining at least four polarity zones the observed polarity zones are roughly layer parallel and may correlate with the illawarra magnetozone at the end of the kiaman if this is so k ar age determinations of an interbedded ash layer kolker and fracasso 1985 suggest that the kiaman superchron ended before 251 4 ma the paleopole located at 51 4 n 126 2 e confirms the reliability of previously reported late permian poles from southwest united states which fall significantly off the late permian segment of the paleomagnetic euler pole pep track of gordon et al 1984 this implies that a more segmented set of small circles is required to model the north american apparent polar wander path diminishing the usefulness of the pep method

paleomagnetism of rocks from burin peninsula newfoundland hypothesis of late paleozoic displacement of acadia criticized the posttectonic st lawrence granite 360 5 ma and associated subvertical dikes have two magnetizations n and h n points north with an intermediate upward inclination declination 005 inclination 50 alpha 95 10 paleopole 12 n 120 e we consider n to have been acquired close to the time of initial cooling the h magnetization points south with low inclination h is also the dominant magnetization in the nearby permo carboniferous spanish room formation h is considered to be an overprint acquired chemically during the kiaman reversed superchron late carboniferous and permian the n magnetization corresponds to a paleolatitude of 31 7 s very different from the near equatorial values expected from the commonly accepted late devonian and earliest carboniferous tournaisian reference paleofields for cratonic north america this is generally consistent with the hypothesis of a southerly displaced acadia proposed by kent and opdyke although the displacement is much greater 30 rather than 15 however we show that all inclinations observed from middle to upper devonian and tournaisian rocks from the craton are not significantly different either from the kiaman paleofield or from the h overprints observed from newfoundland some of these cratonic studies are not very detailed but it is noteworthy that the declination observed in the most comprehensive of them catskill red beds also agrees with the kiaman paleofield and with the h overprints from newfoundland declinations in some other cratonic studies show small departures of only a few degrees with one exception based on limited sampling and two from the colorado plateau which may have been rotated which could be caused by experimental inaccuracies moreover the directions obtained from cratonic studies are 97 reversed as they are in rocks with kiaman magnetizations we suggest therefore that the commonly accepted late devonian to earliest carboniferous reference paleofield for north america is either entirely kiaman in age or has been seriously affected by kiaman overprinting consequently the 30 of southerly displacement which the n magnetization of the st lawrence granite apears to indicate and the 15 invoked by kent and opdyke we consider to be illusory three late precambrian rock units 623 to 606 ma have scattered declinations but similar inclinations yielding paleolatitudes of about 35 the scattered declinations presumable were caused by local pre carboniferous relative rotations there is little or no evidence of kiaman overprinting in precambrian rocks indicating that overprinting is comparatively superficial and is probably related to late paleozoic groundwater circulation

a cratonic middle jurassic paleopole callovian oxfordian stillstand j 2 cusp rotation of the colorado plateau and jurassic north american apparent polar wander 1 paleomagnetic results have been obtained from two sequences of the upper middle jurassic upper callovian summerville formation which are located on the north american craton north central new mexico the thicker of the two sections trujillo exhibits a well defined dual polarity remanence forming a magnetostratigraphy of at least 12 relatively short polarity intervals the high reversal frequency is consistent with that recorded by the oceanic crust at this time the second section romeroville is thin gypsiferous and poorly indurated consequently only a short interval 5 m was successfully sampled the characteristic magnetization was wholly normal polarity in the rapid reversals of the thicker section that thickness of normal polarity is unique and thus forms the basis of magnetostratigraphic correlation between the two sections a cratonic late middle jurassic paleomagnetic pole at 148 2 e 57 3 n k 30 3 95 6 0 was obtained from the larger trujillo sequence despite large confidence limits the small sample population of the romeroville sequence displayed the same pole throughout the western united states the summerville formation is overlain with little or no discordance by the morrison formation although the two stratigraphic units are commonly reported to be separated by an unconformity designated j 5 the new mexico paleopoles of the summerville and overlying lower morrison formations are statistically identical at the 95 confidence level a similar situation exists between summerville and lower morrison paleopoles of the colorado plateau the presence of the same geomagnetic field direction in both the summerville and the lower morrison formations suggests that the j 5 unconformity which separates them may encompass little time perhaps representing only a minor break in deposition the two newly investigated summerville sites are located east of the rio grande rift hence representative of the north american craton their paleopoles are displaced counterclockwise relative to those of the colorado plateau and in amounts consistent with other coeval craton colorado plateau pole pairs together six coeval paleopoles indicate that the colorado plateau has rotated clockwise by about 9 0 3 since late pennsylvanian time the cratonic summerville paleopole also removes the uncertainty over the jurassic north american apparent polar wander path the statistically identical summerville and lower morrison paleopoles form a substantial data set five paleopoles which indicate that na apw traced a path roughly along 60 latitude the five paleopoles stand in stark contrast to the paleopole from the slightly older 5 8 ma moat volcanics spreading rates in the central atlantic do not allow na plate motion that would include both paleopoles positions because the weight of the data five paleopoles from widely spaced localities and different tectonic blocks the moat volcanics pole cannot be representative of north america in the middle jurassic

state dependent strength of sands from the perspective of unified modeling this paper discusses the state dependent strength of sands from the perspective of unified modeling in triaxial stress space the modeling accounts for the dependence of dilatancy on the material internal state during the deformation history and thus has the capability of describing the behavior of a sand with different densities and stress levels in a unified way analyses are made for the toyoura sand whose behavior has been well documented by laboratory tests and meanwhile comparisons with experimental observations on other sands are presented it is shown that the influence of density and stress level on the strength of sands can be combined through the state dependent dilatancy such that both the peak friction angle and maximum dilation angle are well correlated with a so called state parameter a unique linear relationship is suggested between the peak friction angle and the maximum dilation angle for a wide range of densities and stress levels the relationship which is found to be in good agreement with recent experimental findings on a different sand implies that the excess angle of shearing due to dilatancy in triaxial conditions is less than 40 of that in plane strain conditions a careful identification of the deficiency of the classical rowe s and cam clay s stress dilatancy relations reveals that the unique relationship between the stress ratio and dilatancy assumed in both relations does not exist and thereby obstructs unified modeling of the sand behavior over a full range of densities and stress levels

testing the occurrence of late jurassic true polar wander using the la negra volcanics of northern chile abstract true polar wander tpw is the reorientation of the crust mantle system driven by the redistribution of masses in the mantle and on the earth s surface in the ideal case characterization of tpw requires paleomagnetic constraints on the motion of all major plates and independent reconstructions of relative plate positions while such complete datasets are absent for pre mesozoic tpw inferences due to the absence of oceanic plates they are available for the late jurassic 165 145 ma monster shift a 30 amplitude proposed tpw event here we perform paleomagnetic sampling and ar ar geochronology on the la negra volcanics of northern chile producing two new paleomagnetic poles with ages 165 8 1 8 ma 1 84 3 n 0 9 e 95 7 6 n 28 and 152 8 0 8 ma 84 5 n 256 4 e 95 10 8 n 18 by combining these data with other recently published results we compute a net lithospheric rotation of 25 3 7 3 1 at a mean rate of 1 21 0 35 my 1 between 170 and 145 ma with a peak rate of 1 46 0 65 my 1 between 160 and 145 ma these rates are consistent with inferences from the pacific plate implying true whole lithosphere rotation given coherent motion involving the entire lithosphere we conclude that the earth underwent rapid tpw between approximately 165 and 145 ma potentially driven by the cessation of subduction along the western north american margin

palaeomagnetism of the early carboniferous deer lake group western newfoundland no evidence for mid carboniferous displacement of acadia early carboniferous visean to possibly earliest namurian sedimentary rocks of the deer lake group of western newfoundland rest unconformably on grenvillian basement rocks of the canadian shield which form the western border of the early palaeozoic appalachian orogen in addition to magnetically soft magnetizations directed along the present field three families of magnetization directions are found two of them referred to as n north and s south are antiparallel and prefolding and were probably acquired during the depositional process n and s are roughly of equal frequency they have a mean direction irrespective of sign of 0 7 35 2 k 40 95 8 9 and a palaeopole 21 5 n 121 8 e 10 3 6 0 corresponding to a palaeolatitude of 20 6 s this agrees with the palaeolatitude 17 5 s determined from the somewhat older early carboniferous tournaisian terenceville formation of the avalon platform on the eastern side of the appalachian orogen in newfoundland the third magnetization referred to as h roughly horizontal has a mean direction 156 8 13 3 k 37 95 10 1 and a palaeopole 45 4 n 140 3 e 10 3 5 3 corresponding to a palaeolatitude of 7 4 n we interpret this to be an early kiaman latest carboniferous to early permian overprint probably acquired chemically the palaeolatitude determined from the h overprint agrees with that determined from early carboniferous rocks of cratonic north america west of the appalachians therefore we argue early carboniferous palaeofield determinations for cratonic north america have been strongly biased by unremoved kiaman overprints because of this and because of the good agreement between early carboniferous palaeolatitudes obtained from opposite margins of the appalachian orogen we suggest that there is at present no palaeomagnetic evidence for the previously proposed 1500 km displacement from the south of an eastern portion of the appalachians acadia relative to cratonic north america during the carboniferous

paleomagnetic study of the eastern klamath terrane california and implications for the tectonic history of the klamath mountains province paleomagnetic study of permian through jurassic volcanic and sedimentary strata of the eastern klamath terrane has shown the remanent magnetization of many of these rocks to be prefolding and most likely primary similarities in magnetic declinations recorded by coeval strata over a broad area are consistent with the hypothesis that the terrane in general has behaved as a single rigid block paleomagnetic data indicate that the volcanic island arc represented by this terrane the nucleus of the province was facing toward the present southwest during late paleozoic time although its orientation during earlier periods is unknown whether the arc was separated from the north american craton by a small marginal basin or originated far offshore cannot be determined from paleomagnetic data the declination anomalies for both permian and triassic strata are similar average 106 12 so we infer that clockwise rotation of the late paleozoic arc did not begin until latest triassic or earliest jurassic time the arc may have completed its initial rotation with respect to stable north america by middle jurassic time after some retrograde motion the arc was again facing west by the late jurassic by which time some of the more westerly terranes of the province had become attached to the eastern klamath terrane the composite klamath mountains terranes continued to rotate until the final 60 of clockwise rotation was nearly complete by the early cretaceous coincidence of the waning stages of rotation at about 136 ma with the beginning of deposition of the basal great valley sequence onto the klamath basement probably represents the completion of accretion of the klamath mountains terranes to the north american continent nearly all the rotation occurred while the klamath mountains terranes were part of a converging oceanic plate with only about 20 of rotation in mid tertiary time during basin and range extension no data currently available show evidence for any significant latitudinal displacement of any klamath mountains terranes relative to cratonic north america

the chi chi earthquake sequence active out of sequence thrust faulting in taiwan we combined precise focal depths and fault plane solutions of more than 40 events from the 20 september 1999 chi chi earthquake sequence with a synthesis of subsurface geology to show that the dominant structure for generating earthquakes in central taiwan is a moderately dipping 20 to 30 thrust fault away from the deformation front a second subparallel seismic zone lies about 15 kilometers below the main thrust these seismic zones differ from previous models indicating that both the basal decollement and relic normal faults are aseismic

carboniferous through jurassic paleomagnetic data and their bearing on rotation of the colorado plateau small yet systematic differences between paleomagnetic poles derived from strata on the colorado plateau and paleopoles determined from rocks on the north america craton have been interpreted to support the hypothesis of modest post late cretaceous clockwise rotation of the plateau as a quasi rigid body with respect to the craton using an iterative search for the best fit euler pole and rotation angle comparison of the best quality late carboniferous through late jurassic paleomagnetic poles from the colorado plateau and the north america craton gives a cumulative rotation estimate based on a rotation pole at 34 n 105 w of 7 4 3 8 95 confidence limits a similar comparison using subsets of the cratonic database from localities in 1 northeast north america and 2 the craton platform interior give larger 8 8 3 6 and smaller 5 1 3 8 estimates respectively reflecting the fact that poles from localities in northeast north america in particular those from triassic rift basins indicate a larger rotation as concluded in direct pole to pole comparisons the euler pole as determined by the paleomagnetic data only can lie anywhere within a relatively large area that encompasses locations in the western united states previously proposed from geological observations paleomagnetic data and geologic observations together or independently do not support the hypothesis of a large colorado plateau rotation of 11 to 15 if geologically reasonable previous estimates of significant 20 km dextral slip along the eastern margin of the plateau require a position for the euler pole east of the 105 w meridian

nature of the fault jog inferred from a deformed well in the northern chelungpu surface rupture zone related to the 1999 chi chi taiwan ml 7 3 earthquake field investigations show that the northern segments of the chelungpu surface rupture zone form a right stepping echelon with an antidilatational jog where the ground surface was flexure folded during the 1999 chi chi taiwan earthquake a circular well located on the jog was deformed into an ellipsoid shape with the horizontal long axis oriented n70 80 w perpendicular to the strike of the main surface rupture zone deformation increases downward from the top to the bottom in the well based on the geometric characteristics of the surface rupture zone and deformed well we interpret the well as having been deformed by a local nne ssw compressive stress in an antidilatational jog area which was caused by left lateral faulting in the nne ssw trending right stepping chelungpu surface rupture zone this study presents an example that the recognition and identification of jog between two rupture segments has the potential to provide new insight into understanding the controls and mechanisms of rupture initiation and termination during a large earthquake manuscript received 26 july 2000

pre drift extension of the atlantic margins of north america and europe based on paths of permo triassic apparent polar wander summary we reconstruct the relative configuration of north america and europe prior to separation using paths of apparent polar wander apw for the interval 300 to 200 ma the bullard et al 1965 reconstruction closely superimposes the 300 ma points on the two apw paths but leaves the 200 ma points far apart conversely anomaly based reconstructions for later times approximately superimpose the 200 ma ends of the paths but leave the older ends far apart this indicates that separation of the interiors of the two continents began during the interval 300 to 200 ma long before surficial rifting commenced in the late mesozoic this in turn requires pre rift extension in the two continental margins extension appears to have occurred in two phases of approximately equal magnitude but significantly different direction the change in direction occurred at about 200 ma the earlier 300 to 200 ma episode of extension appears to have involved a strong element of sinistral shear based on our preferred reconstruction the total amount of pre rift extension of the two continental margins may have been as much as 1400 km

paleomagnetism of the moenkopi and chinle formations in central new mexico implications for the north american apparent polar wander path and triassic magnetostratigraphy in central new mexico red sedimentary rocks unconformably overlying permian carbonates of the san andres formation have been correlated with the early middle triassic moenkopi and late triassic chinle formations of the colorado plateau paleomagnetic samples from triassic sections exposed on basement cored uplifts along both the east and west side of the rio grande rift near albuquerque yield upon thermal and chemical demagnetization well defined high unblocking temperature dual polarity magnetizations carried by hematite the characteristic magnetization is interpreted as an early acquired chemical remanent magnetization based on a positive intraformational microconglomerate test and bedding parallel magnetization polarity zonation the moenkopi and lowermost chinle formations produced paleomagnetic poles respectively at 57 6 n 100 3 e n 36 sites k 74 1 a95 2 8 and 60 8 n 88 9 e n 17 sites k 130 3 a95 3 1 these data plus previously published and additional results from the underlying permian strata suggest that portions of central new mexico have experienced a small clockwise rotation i e less than 10 similar to that of the colorado plateau with respect to the north american craton the paleomagnetic directions of the chinle formation and related strata in eastern new mexico document about 12 great circle distance of rapid apparent polar wander during mid carnian to late norian times along a track which contains other cratonic poles of similar age we present a preliminary magnetic polarity time scale for the triassic that incorporates the present new mexico data and previously published data mostly form continental red bed sequences this magnetic polarity scale provides a basic framework which can be tested with future data from triassic sections where additional biostratigraphic control exists

rotation of the colorado plateau an analysis of paleomagnetic data hamilton 1981 has proposed that the colorado plateau has rotated clockwise relative to cratonic north america by a total of 5 7 since mid cretaceous time a rotation this small is unresolvable by most paleomagnetic studies in this paper we present a new method for simultaneously comparing all of the available paleomagnetic data from on and off the plateau in order to estimate rotation of the colorado plateau our new method examines the effect of various hypothetical plateau rotations on the dispersion of data fit to a smooth model of apparent polar wander a maximum likelihood criterion is used to estimate the amount of rotation confidence limits are found using an f ratio test assuming the plateau craton euler pole to be located at 37 n 103 w we applied this method to north american paleomagnetic data of carboniferous to cretaceous age the best paleomagnetic estimate of the rotation is 3 9 clockwise with a 95 confidence interval of 1 4 6 6 clockwise in good agreement with geologic estimates of rotation because of the dependence of published north american apparent polar wander paths on data from the colorado plateau the existence of such a rotation requires a minor revision in the apparent polar wander path the largest change being a 3 9 eastward shift of the cusp connecting the carboniferous permian triassic and jurassic cretaceous tracks

velocity field of gps stations in the taiwan area abstract the 131 stations of the taiwan gps network were surveyed 4 6 times from 1990 to 1995 with dual frequency geodetic receivers the standard deviation of an observed baseline length with its linear trend removed is in the range of 6 10 mm for a 3 120 km long baseline the average rates of length change for all baselines of the network and those from nine continuously monitoring permanent stations are used in a least squares adjustment to estimate the velocities of the gps stations relative to paisha penghu situated at the chinese continental margin to the south of fengping in the northern coastal range the velocity vectors of stations in lanhsu lutao and the coastal range trend in the directions of 306 322 with rates of 56 82 mm yr in contrast there is a dramatic decrease in the rates to the north of fengping this may be caused by the motion along the ne sw trending thrusts which obliquely cut the northern coastal range a discontinuity of about 30 mm yr in the rates along with a remarkable change in the directions of station velocity is observed across the longitudinal valley then the moving directions gradually shift to the west for the stations in the western foothills in the kaohsiung pingtung coastal area the station velocities are even directed toward the southwest to the north of the peikang high the velocity vectors of the stations change direction from the west gradually to the north and finally to the east and southeast significant nw se extensional deformation is found in the ilan plain and northern taiwan in general the pattern of the velocity field for gps stations in the taiwan area is quite consistent with the directions of present day tectonic stress

surface rupture of 1999 chi chi earthquake yields insights on active tectonics of central taiwan the 1999 chi chi earthquake was caused by rupture of the chelungpu fault one of the most prominent active thrust faults of taiwan this largest of taiwan s historical fault ruptures broke the surface for over 90 km at the western base of the rugged mountain range a short right lateral tear extended southwestward from the southern end of the chelungpu fault and a complex assemblage of shallow folds and faults ran northeastward from the northern end vertical offsets averaged about 2 m along the southern half of the chelungpu fault and about 4 m along the northern half and offsets of 5 to 7 m were typical along the northern part of the major thrust the sinuous nature of the surface trace is consistent with seismographic data that indicate a dip of about 30 the 1999 rupture draws attention to the fact that this active fault system is highly segmented and that this segmentation influences the characteristics of seismic ruptures active faults to the south north and west of the chelungpu fault have distinctly different characteristics faults to the south and north broke the surface during earthquakes in 1906 and 1935 the active changhua fault to the west a blind thrust similar in length to the chelungpu has not ruptured in the historical period and should be considered a prime candidate for generating a future earthquake manuscript received 4 october 2000

rotation of the colorado plateau consistent displacement of its paleomagnetic pole positions suggests that the colorado plateau has rotated with respect to cratonic north america since pole positions derived from the colorado plateau form a significant portion 35 of the pre cretaceous population of north american paleomagnetic data the rotation has significant impact on the continent s data base comparison of paleopoles from nearly coeval strata on and off the plateau shows that the plateau pole positions are always displaced clockwise from non plateau counterparts in amounts varying from 9 to 14 the time periods for which such comparisons are possible are the late triassic the early triassic the late pennsylvanian and the middle to late devonian other time periods contain data which appear to be consistent with a clockwise displacement of plateau pole positions but poor age control rapid apparent polar wander and late paleozoic remagnetization all hamper comparisons only one of the time periods examined the permian does not show net rotation of the plateau observation of the rotation of the plateau has been obscured by the fact that a great many of the plateau poles are from times of rapid apparent polar wander apw and the displacement effected by rotation parallels the major apw trends this renders the discrepancy of plateau poles relatively inconspicuous and is only readily observable between poles from the same age strata the rotation of the plateau has particular impact on the early and middle mesozoic paleopoles to which the plateau had contributed heavily in particular for the late jurassic plateau poles are the only data available for the analysis of western north america allochthonous terrane movements

a vertical exposure of the 1999 surface rupture of the chelungpu fault at wufeng western taiwan structural and paleoseismic implications for an active thrust fault we mapped and analyzed two vertical exposures exposed on the walls of a 3 to 5 m deep 70 m long excavation and a smaller 3 m deep 10 m long excavation across the 1999 rupture of the chelungpu fault the primary exposure revealed a broad anticlinal fold with a 2 5 m high west facing principal thrust scarp contained in fluvial cobbly gravel beds and overlying fine grained overbank deposits sequential restoration of the principal rupture requires initial failure on the basal east dipping thrust plane followed by wedge thrusting and pop up of an overlying symmetrical anticline between two opposing secondary thrust faults net vertical offset is about 2 2 m across the principal fault zone from line length changes we estimate about 3 3 m of horizontal shortening normal to fault strike the ratio of these values yields a total slip of 4 0 m and an estimate of about 34 for the dip of the fault plane below the excavation this value is nearly the same as the 35 average dip of the fault plane from the surface to the hypocenter restoration of the exposed gravelly strata and adjacent overbank sediments deposited prior to the 1999 event around the principal rupture suggests the possible existence of a prior event a buried 30 m wide anticlinal warp beneath the uplifted crest of the 1999 event is associated with three buried reverse faults that we interpret as evidence for an earlier episode of folding and faulting in the site the prior event is also recorded in the smaller excavation which is located 40 m south and is oriented parallel to the larger excavation radiocarbon dating of samples within the exposed section did not place tight constraints on the date of the previous event available data are interpreted as indicating that the previous event occurred before the deposition of the less than 200 14 c yr b p overbank sands and after the deposition of the much older fluvial gravels we interpret the previous event as the penultimate event relative to the 1999 chi chi earthquake we estimated the long term slip rate of the chelungpu fault to be 10 15 mm yr during the last 1 ma based on previously published retrodeformable cross sections this rate is however significantly higher than geodetic rates of shortening across the chelungpu thrust where two pairs of permanent global positioning system stations suggest 7 10 mm yr of shortening across the fault given the 4 m of average slip the long term slip rate yields an interseismic interval of between 267 and 400 yr for the chelungpu fault

paleomagnetic results from the late carboniferous early permian casper formation implications for northern appalachian tectonics abstract paleomagnetic samples were collected from 190 m of the late carboniferous early permian casper formation in southeastern wyoming a total of 549 samples was drilled near the vicinity of horse creek station at an average stratigraphic interval of 33 cm all samples were reversely magnetized rock magnetic analyses indicate that the primary carrier of remanence in the formation is hematite a selection criterion applied to the partial demagnetized data restricted the sample population to 233 resulting in a paleomagnetic north pole located at 47 4 n 127 4 e p 0 7 m 1 4 the casper pole agrees well with other late carboniferous early permian poles for cratonic north america the tight clustering of these paleomagnetic poles suggests that little apparent polar motion with respect to north america occurred during this time comparing the stable north american poles with paleomagnetic poles from late carboniferous early permian strata of the new england canadian maritime region acadia indicates that this region did not reach its present position relative to north america until at least the early permian

the pictou red beds pennsylvanian pole could phanerozoic rocks in the interior united states be remagnetized the pennsylvanian red beds of the pictou group on prince edward island canada retain a stable remanence in hematite that gives a paleomagnetic pole position at 42 n 131 e dp 2 dm 4 positive conglomerate and contact tests along with dual polarities show that the remanence is primary and of stephanian age 286 306 ma for elongated pole distributions through time bingham statistics is more suitable than fisher statistics bingham statistics shows that there is a consistent discordance in the pennsylvanian to late triassic poles from the appalachian orogenic belt and interior craton of north america this discordance is most readily explained by younger partial remagnetization or possibly complete remagnetization in many rock units especially red beds of the interior craton bingham analysis also shows that there is a significant difference between the colorado plateau and interior craton that is most readily explained by a laramide 5 5 2 9 clockwise rotation of the plateau relative to the craton

design of driven piles in sand estimation of the axial capacity of piles driven into sand involves considerable uncertainty and design rules are generally not consistent with the physical processes involved this paper reviews current understanding of the factors that determine the axial capacity of piles driven into sand and outlines a new framework for design which takes account of the physical processes is consistent with the existing database of load test results and is sufficiently flexible to permit refinement as new data become available it allows for the effects of confining stress on the frictional and compressibility characteristics of sand and hence on end bearing capacity in keeping with field observations shaft friction is assumed to degrade with driving of the pile past a particular location from an initial maximum value linked to the local end bearing capacity the resulting design approach is compared with field data and effects of factors such as the direction of loading are discussed l estimation de la resi

complexity of the coseismic rupture for 1999 chi chi earthquake taiwan from inversion of gps observations abstract based on the elastic half space model and the layered earth model we use global position system observations to invert the distribution of coseismic slip along the chi chi earthquake rupture bending fault planes are introduced in our models the results indicate that the shear slip model can not fit horizontal and vertical coseismic displacements simultaneously unless the fault normal motion tensile component is considered in the inversion we interpret that the fault normal motion reflects the roughness of fault planes demonstrating the complexity of the rupture in property and geometry the distribution of the fault normal motion appropriately fits with the complexity of surface coseismic rupture from the field investigation and the distribution of the strike dip slip of coseismic rupture is similar to the results from inversion of the teleseismic data

a successful rock magnetic technique for correctng paleomagnetic inclination shallowing case study of the nacimiento formation new mexico the accuracy of an inclination shallowing correction technique was tested on the remanence of the paleocene nacimiento formation the correction assumes that changes in paleomagnetic inclination during deformation will be directly related to changes in remanence anisotropy the remanence anisotropy is also a function of the magnetic anisotropy of the individual magnetic grains the nacimiento formation was selected for this test because its inclination is shallow by 7 to 8 and previous paleomagnetic studies indicate that it has a primary remanence with a well constrained age and that magnetic overprints are minimal tectonics are an unlikely explanation for its shallow inclination these conditions allow the inclination corrected remanence to be compared to a well defined paleomagnetic pole to determine the accuracy of the technique the characteristic remanence from 20 nacimiento sites was isolated by alternating field and thermal demagnetization and was corrected using the anisotropy of anhysteretic remanence measured for three to five samples per site individual particle anisotropy was determined by drying a magnetic separate epoxy mixture in a 35 50 mt magnetic field laboratory compaction of disaggregated nacimiento material was also used to yield an effective individual magnetic particle anisotropy the two techniques resulted in similar values for individual particle anisotropy and similar inclination shallowing corrections both corrections were successful and the inclination corrected formation mean direction was indistinguishable from the direction predicted by north america s paleocene paleopole the inclination shallowing correction technique reported here should be applied routinely in paleomagnetic studies provided that the remanence is a primary depositional remanence that has been affected only by syndepositional or early postdepositional processes e g compaction

widespread inclination shallowing in permian and triassic paleomagnetic data from laurentia support from new paleomagnetic data from middle permian shallow intrusions in southern illinois usa and virtual geomagnetic pole distributions abstract recent paleomagnetic work has highlighted a common and shallow inclination bias in continental redbeds the permian and triassic paleomagnetic records from laurentia are almost entirely derived from such sedimentary rocks so a pervasive inclination error will expectedly bias the apparent polar wander path of laurentia in a significant way the long standing discrepancy between the apparent polar wander paths of laurentia and gondwana in permian and triassic time may be a consequence of such a widespread data pathology here we present new middle permian paleomagnetic data from igneous rocks and a contact metamorphosed limestone from cratonic laurentia the exclusively reversed middle permian magnetization is hosted by low ti titanomagnetite and pyrrhotite and yields a paleomagnetic pole at 56 3 s 302 9 e a95 3 8 n 6 this pole which is unaffected by inclination shallowing suggests that a shallow inclination bias may indeed be present in the laurentian records to further consider this hypothesis we conduct a virtual geomagnetic pole distribution analysis comparing theoretical expectations of a statistical field model tk03 gad against published data sets this exercise provides independent evidence that the laurentian paleomagnetic data is widely biased likely because of sedimentary inclination shallowing we estimate the magnitude of this error from our model results and present and discuss several alternative corrections

slip distribution and tectonic implication of the 1999 chi chi taiwan earthquake we report on the fault complexity of the large m w 7 6 chi chi earthquake obtained by inverting densely and well distributed static measurements consisting of 119 gps and 23 doubly integrated strong motion records we show that the slip of the chi chi earthquake was concentrated on the surface of a wedge shaped block the inferred geometric complexity explains the difference between the strike of the fault plane determined by long period seismic data and surface break observations when combined with other geophysical and geological observations the result provides a unique snapshot of tectonic deformation taking place in the form of very large 10m displacements of a massive wedge shaped crustal block which may relate to the changeover from over thrusting to subducting motion between the philippine sea and the eurasian plates

late cretaceous paleomagnetism of the tucson mountains implications for vertical axis rotations in south central arizona the tucson mountains of southern arizona are the site of an upper cretaceous caldera from which the rhyolitic cat mountain tuff was erupted at about 72 ma two magnetic units within the cat mountain tuff are distinguished by paleomagnetic data in both the northern and southern tucson mountains the resurgent amole pluton 72 ma in the northern tucson mountains was emplaced soon after eruption of the cat mountain tuff but cooled and was magnetized after northeastward tilting 50 85 of the adjacent caldera fill sequence petrologic and paleomagnetic data indicate that the lower magnetic unit of the cat mountain tuff caps the silver bell mountains to the northwest a previous paleomagnetic investigation n 34 indicates that the silver bell mountains have been rotated clockwise 30 16 95 confidence level about a vertical axis relative to cratonic north america a similar paleomagnetic study of upper cretaceous volcanic volcaniclastic and intrusive units in the tucson mountains n 26 indicates that these rocks have been rotated 7 14 clockwise relative to stable north america a direct comparison of paleomagnetic directions for the lower unit of the cat mountain tuff shows a 17 10 clockwise rotation between the silver bell mountains and the tucson mountains which supports the relative accuracy of the absolute rotations determined for these two mountain ranges preliminary paleomagnetic directions for middle tertiary units from the silver bell and tucson mountains are consistent with clockwise rotation having occurred prior to deposition of these rocks clockwise rotation of crustal blocks in southern arizona likely was associated with strike slip movement on major northwest trending faults in the region and this movement may have been related to oblique subduction of oceanic plates along the western continental margin in late cretaceous and early tertiary time the available paleomagnetic data indicate that rocks in southern arizona have not remained unrotated with respect to north america since late cretaceous time and that vertical axis rotations may have played an important role in the region during laramide deformation

paleomagnetism of three upper jurassic ash flow sheets in southeastern arizona implications for regional deformation a sequence of three petrologically distinct upper jurassic ash flow tuffs has been sampled for palcomagnetic analysis in the huachuca mountains canelo hills and mustang mountains of southeastern arizona site mean palco magnetic directions for these units indicate significant ver tical axis rotations between sampling localities 15o 40o and a comparison of these and previously published data with reference directions from the colorado plateau implies that the jurassic rocks in southeastern arizona have been rotated in a clockwise sense with respect to stable north america these new data are consistent with palcomagnetic results showing clockwise rotation of upper cretaceous rocks in south central arizona and further indicate that studies of north american apparent polar wander based on rocks from southern arizona could be subject to error the clockwise rotations in southern arizona are likely related to late cretaceous and early tertiary strike slip movement on northwest trending high angle faults in the region and this movement may be linked to an increase in oblique farallon north american plate conver gence at that time

discussion of shaft resistance of single vertical and batter piles driven in sand by a hanna and t q nguyen the paper has presented an experimental study on the shaft resistance of driven piles in sand a topic of interest in foundation design to better validate the results and analyses several points are raised here for further clarification fig 6 in the original paper presents the average ratio as a function of pile diameter for different pile length diameter sdm fmdave ratios where dm is the mobilized angle of friction between pile shaft and surrounding soil and fm is the mobilized friction angle of soil it is indicated that for a specific value of l d the ratio sdm fmdave will increase linearly with the pile diameter whereas for a given pile diameter it will decrease as the ratio l d increases the proposed relationships look quite neat however care should be exercised in their use because they are established on the basis of limited experimental data that were obtained under different test conditions the different test conditions may lie in the tested materials test methods and particularly test scales i e small scale model tests or prototype field tests for example the data point from vesic s 1967 work shown in the graph is for a pile having a diameter of 450 mm and a length diameter ratio of 33 35 whereas the data points from the authors investigation are for piles having a diameter of 76 mm and the length diameter ratios of 10 and 20 respectively only two data points from the authors work were included both for the 76 mm model piles the test results for the 38 mm model piles were however excluded from their fig 6 it is noticed from their fig 9 the case of a 0 that the experimental data for the 38 mm model piles seem not to support the relationships in the authors fig 6 as shown here in fig 1 in deriving the results in the authors tables 3 and 4 the mobilized angle of shearing resistance of the sand fm was assumed to be equal to f and a specific value f 39 was used in the computation for the 76 mm model piles in the authors fig 10 however f 38 1 was assumed in the computation for the 38 mm model piles the justification for choosing these values of friction angle is not clear it is indicated in the paper that the sand deposit has a relative density of 65 3 this corresponds to a void ratio of 0 396 if one takes the maximum and minimum void ratios in the authors table 1 as a reference the values of the friction angle used for the computation however imply that the void ratio should be around 0 67 if the triaxial test results in the authors fig 2 serve as a reference source moreover it is noted that the void ratios covered by fig 2 seem to be far beyond the range set by emax 0 52 and emin 0 33 although emax and emin are index void ratios obtained by standard procedures and are not necessarily the absolute maximum and minimum void ratios in reality in the authors table 3 the average coefficient of earth pressure acting on the pile shaft kssaved was obtained as 8 04 and 7 28 for l d 10 and 20 respectively following the method of analysis and according to the design charts given in the authors fig 7 ksaved should be close to 10 for vertical piles sa 0 d in the sand having the angle of shearing resistance f 39 it then follows according to kssaved rksaved that the reduction factor r takes the

a geomagnetic polarity timescale for the permian calibrated to stage boundaries the reverse polarity kiaman superchron has strong evidence for at least three or prob ably four normal magnetochrons during the early permian normal magnetochrons are during the early asselian base ci1r 1n at 297 94 0 33 ma late artinskian ci2n at 281 24 2 3 ma mid kungurian ci3n at 275 86 2 0 ma and roadian ci3r an at 269 54 1 6 ma the mixed polarity illawarra superchron begins in the early wordian at 266 66 0 76 ma the wordian capitanian interval is biased to normal polarity but the basal wuchiapingian begins the beginning of a significant reverse polarity magnetochron lp0r with an overlying mixed polarity interval through the later lopingian no significant magnetostratigraphic data gaps exist in the permian geomagnetic polarity record the early cisuralian magnetochrons are calibrated to a succession of fusulinid zones the later cisuralian and guadalupian to a conodont and fusulinid biostratigraphy and lopingian magnetochrons to conodont zonations age calibration of the magnetochrons is obtained through a bayesian approach using 35 radiometric dates and 95 confidence intervals on the ages and chron durations are obtained the dating control points are most numerous in the gzhelian asselian wordian and changhsingian intervals this significant advance should provide a framework for better correlation and dating of the marine and non marine permian

coseismic and postseismic fault slip for the 17 august 1999 m 7 5 izmit turkey earthquake we use global positioning system gps observations and elastic half space models to estimate the distribution of coseismic and postseismic slip along the izmit earthquake rupture our results indicate that large coseismic slip reaching 5 7 meters is confined to the upper 10 kilometers of the crust correlates with structurally distinct fault segments and is relatively low near the hypocenter continued surface deformation during the first 75 days after the earthquake indicates an aseismic fault slip of as much as 0 43 meters on and below the coseismic rupture these observations are consistent with a transition from unstable episodic large earthquakes to stable fault creep sliding at the base of the seismogenic zone

representative styles of deformation along the chelungpu fault from the 1999 chi chi taiwan earthquake geomorphic characteristics and responses of man made structures the chi chi earthquake provides dramatic evidence of the damaging effects of surface ground deformation to buildings lifelines and other facilities much of the building damage is associated with surface faulting and folding along the chelungpu thrust fault our detailed surveying at representative sites along the fault shows that the rupture commonly is a relatively simple 1 to 4 m high scarp with minor hanging wall deformation and localized but severe uplift folding and graben formation along the scarp crest for individual scarps the width of deformation is about 10 to 20 times the net vertical displacement distributed secondary faulting and folding on the hanging wall occurred as much as 350 m from the primary fault near the northern end of the rupture growth of a pre existing 1 km wide late quaternary anticline produced severe ground rupture along multiple thrusts and backthrusts but only minor tilting between fault strands the pattern of building damage coincides with the pattern of geologic deformation with severe damage along large fault scarps and lesser but still significant damage attributable to distributed secondary surface deformation on the hanging wall rupture related building damage on the footwall occurred next to the prerupture fault trace where the hanging wall bulldozed onto the footwall the width of this damage zone is related to the local horizontal shortening along the fault and generally is less than 10 m building zonation along reverse faults should account for this pattern of surface deformation in addition buildings with massive foundations locally influenced the style and location of near surface deformation producing variations in fault strike or accentuated secondary deformation on the hanging wall manuscript received 15 january 2001

palaeozoic palaeogeography of laurentia and its margins a reassessment of palaeomagnetic data summary the recognition of wide spread remagnetization events during the palaeozoic has greatly added to the problems of reconstructing the palaeozoic drift history of laurentia and in recent years increasing emphasis has been placed on evaluating the relative reliability of palaeomagnetic data in an attempt to overcome some of these difficulties in the light of these developments a revised apparent polar wander apw path has been constructed following a rigorous evaluation of all published palaeozoic palaeomagnetic data from laurentia and its palaeozoic margins i e north america northern britain northern ireland and greenland the use of data from the laurentian margins when corrected for the opening of the north atlantic has resulted in a revision of previously published palaeozoic palaeogeographic reconstructions and these new palaeogeographic reconstructions for laurentia are in good agreement with faunal and lithological data a comparison of this new apw path with other apw paths from avalonia and baltica reveals a very close agreement between all three paths from mid silurian times onwards indicating that the closure of the iapetus ocean and the tornquist sea was essentially complete by this time laurentia occupied equatorial latitudes from cambrian to early silurian times and its drift history was dominated by anti clockwise rotation up to 0 7 ma 1 and low latitudinal drift rates up to 3 cm yr 1 late silurian and early devonian times were characterized by increased rates of anti clockwise rotation up to 1 3 ma 1 and southerly directed latitudinal drift rates up to 13 cm yr 1 of the continent which place laurentia in moderate southerly latitudes 30 40 s by the early devonian these palaeolatitudes are comparable with those from avalonia and baltica for this time indicating that the amalgamation of eurmerica had already taken place by late devonian time laurentia had begun to drift north rates up to 5 cm yr 1 towards equatorial latitudes by mid carboniferous time

concordant paleolatitudes from ophiolite sequences in the northern california coast ranges u s a abstract paleomagnetic data have been obtained from two ophiolite sequences in the northern california coast ranges from mount diablo in the san francisco bay area and from potter valley north of clear lake the ophiolite exposed at mount diablo is part of the late middle to late jurassic coast range ophiolite and that exposed near potter valley is late jurassic to perhaps early cretaceous in age and occurs within the franciscan assemblage data from the sheeted dike complex at mount diablo show these rocks to be strongly overprinted probably following uplift and erosion of the ophiolite samples whose primary remanent magnetization seems to be recovered yield a mean paleomagnetic pole at 30 7 n 159 5 e with 95 5 6 a comparison of this pole with the jurassic apparent polar wander path for north america indicates that the ophiolite has rotated 45 7 counterclockwise relative to the craton and has not been latitudinally displaced the diabase and pillow basalt in potter valley have not been strongly overprinted and data from those rocks yield a paleomagnetic pole at 79 0 n 61 5 e with 95 6 4 this result indicates that the ophiolite at potter valley has rotated approximately 29 8 clockwise and has undergone little or no latitudinal displacement because of the predominantly northeastward transport of oceanic plates converging with the western margin of north america since middle mesozoic time the absence of appreciable northward displacement of either ophiolite fragment indicates that both formed close to the continental margin

rheologic control of buoyancy driven extension of the rio grande rift extensional deformation in the western united states particularly in the basin and range has frequently been ascribed to buoyancy forces arising from horizontal density contrasts the rio grande rift is subject to buoyancy forces 2 3 times larger than those in the tectonically active northern basin and range suggesting that it should be extending considerably faster than the northern basin and range yet paradoxically is extending more slowly using a thin viscous sheet model we test the hypothesis that regions surrounding the rio grande rift which have rheologically stronger lithosphere resist deformation and reduce the rate of extensional deformation in the rio grande rift to values consistent with observations we find that these regions must have lithospheric strength at least twice that of the rio grande rift this modest contrast in lithospheric strength is consistent with independent estimates derived from observations of heat flow seismicity and crustal thickness

rupture mechanism and interface separation in foam rubber models of earthquakes a possible solution to the heat flow paradox and the paradox of large overthrusts abstract spontaneous stick slip along the interface between stressed foam rubber blocks is a simple analog of earthquake rupture and stick slip results from this model are used to elucidate the role of normal stress variations along the interface in the stick slip process observations indicate significant normal interface vibrations and separation during slip suggesting that dynamic changes in normal stress rather than a drop in the coefficient of friction may control stick slip as suggested for example by tolstoi oden and martins and brune and co workers observations of particle trajectories indicate that stick slip shear motion is associated with various degrees of fault separation for an asymmetric model the motion is consistent with slipping motion of the type suggested by schallamach and price for a symmetric model the motion is similar to that suggested by comninou and dundurs if interface waves of this type involving separation during slip occur in earthquakes they may be a solution to the heat flow paradox since a major part of the slip occurs during separation and during low normal stress thus frictional heat generation is reduced normal interface vibrations during stick slip may explain the high corner frequency of p wave spectra and the generally high levels of p wave spectra beyond the corner frequency schallamach comninou type waves are consistent with the partial stress drop abrupt locking self healing models of brune and heaton

paleomagnetism of the late pennsylvanian and permian a test of the rotation of the colorado plateau coeval formations on both the north american craton and the colorado plateau have been sampled for the comparison of paleomagnetic pole positions the latest pennsylvanian earliest permian laborcita and the early permian abo formations of eastern new mexico and the latest pennsylvahian wescogame formation of the supai group of northwestern arizona were studied thermal demagnetization demonstrated that the majority of the samples in all three formations were remarkably free of significant secondary overprinting computation of pole positions used only samples which exhibited univectorial decay to the origin over at least three or more temperature steps the paleopole of the wescogame formation is 119 2 e 47 3 n n 45 samples k 41 6 dp 1 7 and dm 3 4 the laborcita formation paeopole is 130 6 e 41 8 n n 114 samples k 42 9 dp 1 0 and dm 2 0 and the overlying abo formation paleopole is 118 3 e 49 7 n n 84 samples k 54 9 dp l l and dm 2 1 comparison of the wescogame laborcita abo and other paleopoles of this age shows the plateau poles displaced clockwise confirming a clockwise rotation of the colorado plateau with respect to stable north america the new data could be interpreted to indicate a 7 4 rotation of the plateau however the direction of displacement of pennsylvanian and permian paleopoles is significantly different from that exhibited by the early and the late triassic poles the current body of data is best fit by not one but two rotations and suggests that the colorado plateau rotated in late permian time prior to a later presumably laramide rotation separate calculations of the late permian rotation from late pennsylvanian data and from early permian data both indicate a rotation of about 9 5 2 7 northward and 5 4 2 7 westward this study thus suggests that at least two rotations of the colorado plateau have occurred prior to the most recent rotation involving the opening of the rio grande rift this history of rotations implies a quasi independent plate motion behavior for the colorado plateau microplate

estimation of depth dependent fault slip from measured surface deformation with application to the 1906 san francisco earthquake we estimate the depth distribution of slip in the 1906 san francisco earthquake by applying new inversion methods to triangulation data collected near point arena california modeling crustal deformation from antiplane slip in a half space we define three regularizing functionals to produce minimum norm inverse problems these functionals measure components of the coseismic change in elastic strain energy stress magnitude and stress variability and are all quadratic in slip orthogonality conditions define finite dimensional representations of slip estimates coefficients of basis functions in these representations are estimated by damped least squares with damping parameter and faulting depth chosen by cross validation the best resulting estimates exhibit right lateral slip to depths of 15 to 20 km with about 6 m of surface slip this is consistent with directly observed offsets of cultural landmarks at the surface and indicates deeper faulting than has been previously inferred for the 1906 earthquake

a geomagnetic polarity timescale for the carboniferous the geomagnetic polarity pattern for the carboniferous is incompletely known but with the best resolved parts in the serpukhovian and bashkirian hence data from both igneous and sedimentary units are also used in an additional polarity bias evaluation in the tournaisian to mid visean interval polarity is mainly derived from palaeopole type palaeomagnetic studies allowing identification of polarity bias chrons seven polarity bias chrons exist in the mississippian mi1nb to mi4nb with an additional 33 conventional magnetochrons and submagnetochrons mi4r to mi9r the moscovian and gzhelian polarity is best resolved in magnetostratigraphic studies from the donets basin and the southern urals dispute about the reliability of these data is ill founded since an assessment of supporting data from palaeopole type studies suggests that these datasets currently provide the best magnetic polarity data through the pennsylvanian polarity bias assessment indicates a normal polarity bias zone in the kasimovian in the pennsylvanian there are 27 conventional magnetochrons and submagnetochrons pe1n to ci1r and one normal polarity bias chron pe8nb the kiaman superchron begins in the mid bashkirian with clear data indicating brief normal polarity submagnetochrons within the superchron the magnetochron timescale is calibrated using 31 u pb zircon dates and a quantitative bayesian based age scaling procedure

paleomagnetism of the chinle and kayenta formations new mexico and arizona paleomagnetic data were obtained from 22 sites 6 10 samples site in the upper shale member of the chinle formation 43 sites in the owl rock member of the chinle formation and 35 sites in the kayenta formation thermal demagnetization and data analyses indicate that within site dispersion is an important criterion for selecting sites which retain a high unblocking temperature characteristic remanent magnetization chrm site mean directions define at least four antipodal polarity zones within each member formation suggesting the chrm was acquired soon after deposition fifteen site mean virtual geomagnetic poles vgps from the upper shale member of the chinle formation yield an early norian paleomagnetic pole position of 57 4 n 87 8 e k 60 a95 5 0 eighteen site mean vgps from the owl rock member of the chinle formation yield a middle norian paleomagnetic pole position of 56 5 n 66 4 e k 183 a95 2 6 twenty three site mean vgps from the kayenta formation yield a pliensbachian pole position of 59 0 n 66 6 e k 155 a95 2 4 combined with paleomagnetic poles from the moenave formation and the shinarump member of the chinle formation these data record 30 m y of north american apparent polar wander apw within a regional stratigraphic succession during the camian and norian stages of the late triassic chinle poles progress westward during the hettangian through pliensbachian stages of the early jurassic the pattern of apw changed to an eastward progression even after correction for 4 clockwise rotation of the colorado plateau a sharp comer in the apw path j1 cusp is resolved near the pole from the hettangian sinemurian 200 ma moenave formation 59 4 n 59 2 e amongst other implications the sharp change in the apw path at the j1 cusp implies an abrupt change from counterclockwise rotation of pangea prior to 200 ma to clockwise rotation thereafter

postseismic deformation following the 1989 m 7 1 loma prieta california earthquake postseismic deformation along a 90 km profile bisecting the projected surface trace of the coseismic rupture of the 1989 loma prieta earthquake has been monitored by frequent gps surveys for 3 3 years following the earthquake in addition to the expected deformation associated with secular strain accumulation on the san andreas and calaveras faults deformation associated with postseismic readjustment has been detected most of that deformation can be attributed to 1 5 m right lateral and 0 9 m reverse postseismic slip on a 5 km wide downdip extension of the loma prieta rupture in addition there seems to be a 0 1 m postseismic collapse of the loma prieta rupture zone in the direction perpendicular to the plane of the rupture the fault normal n48 e surface displacements plotted as a function of time exhibit a curvature suggesting a relaxation time of about 1 4 years similar plots of the fault parallel n42 w displacement components do not exhibit significant curvature presumably the deformation shown in those plots is dominated by secular strain accumulation along the san andreas and calaveras faults rather than postseismic relaxation

preseismic deformation and coseismic displacements associated with the 1999 chi chi taiwan earthquake the destructive 1999 chi chi taiwan earthquake m w 7 5 produced an approximately 100 km long surface rupture mostly along the previously recognized north south trending chelungpu fault preseismic deformation in central taiwan is realized from annually repeated global positioning system gps data acquired during the 1992 1999 period the total wnw ese shortening rate in the vicinity of the epicentral region that is from the west coast to the western boundary of the central range is up to 25 mm yr the crustal deformation before the chi chi earthquake was essentially a uniaxial compressional strain of 0 36 strain yr in the direction of 114 the gps measurements taken 0 2 2 7 yr before and within 3 months after the mainshock were utilized to estimate the coseismic displacements horizontal movements of 1 1 9 1 m in the nw nnw directions are observed on the hanging wall eastern side of the fault there is a northward increasing trend in the magnitude of the displacement vectors and a dramatic change in the direction of about 50 toward the east along the fault strike in contrast much smaller se see movements of 0 1 1 5 m are found on the footwall western side of the fault the gps data show 2 4 10 1 m of total horizontal offsets across the chelungpu fault vertical offsets of 1 2 4 4 m with the eastern side up are also observed along the surface rupture the uplift on the hanging wall decreases rapidly toward the east it becomes subsidence at sun moon lake and in the puli wushe area the stations on the footwall show subsidence of 0 02 0 26 m the width of the uplift zone increases from about 10 km in the south to approximately 30 km in the north manuscript received 13 october 2000

present day motion and deformation of the colorado plateau 1 we analyze data from continuous gps stations in the southwestern united states and present the first observations of a systematic motion of the colorado plateau region relative to a north america fixed reference frame westward velocities increase rapidly across the wasatch fault system which comprises the plateau s northwestern margin but increase more gradually between the great plains and the mojave desert province we detect no significant extension across most of the rio grande rift but extension could be as high as 0 5 mm yr 1 along its southernmost portion we suggest that the motion between the plateau and stable continent may partly be accommodated instead across the jemez lineament only when we consider gps velocities within the plateau s center do the data support a rigid body rotation of 0 103 ma 1 around a pole in the northern rocky mountains we conclude that active extension has encroached into the plateau from both the east and west

fault rupture process of the 20 september 1999 chi chi taiwan earthquake we modeled the source rupture process of the 20 september 1999 chi chi taiwan earthquake mw 7 6 using an integrated dataset of near field strong ground motion and global positioning system observations this large inland thrust earthquake produced a surface break more than 80 km long along the chelungpu fault the event triggered over 700 strong motion instruments on the island of tai wan this large ground motion dataset provides the best opportunity to date to un derstand earthquake source processes in a major earthquake first we develop a 3d curved fault model consistent with the surface rupture data and aftershock distribu tion the subsurface fault plane has an average dip of 30 to the east and measures 96 km along strike and 40 km down dip next we applied a genetic algorithm to reconstruct the source rupture process the inversion results indicate a complex slip distribution with a generally broad triangular shaped slip zone that points down dip most of the displacement occurred over a 15 km depth range in the northern segment of the fault with the maximum slip exceeding 20 m the average slip is 3 8 m and the average rupture velocity is 2 6 km sec the mainshock slip distribution clearly complements the aftershock location distribution slip rise times range from 8 to 6 sec over most of the primary slip area and decrease slightly as rupture propagates south to north this relatively long rise time resulted in a low dynamic stress drop and low peak ground accelerations observed at surface stations the rake angle rotates across the rupture plane from predominantly dip slip in the southern segment to mostly left lateral oblique slip in the north we find that geometrical irregularities over the fault plane played an important role in controlling the temporal rupture behavior at fault bends rupture tends to decelerate the rake angle rotates to a near dip slip orientation and the shear stress increases as rupture tries to overcome the bending barriers the total seismic moment from our inversion is 2 9 10 27 dyne cm in good agreement with the harvard and the u s geological survey moment estimation

paleomagnetism of middle proterozoic diabase sheets from central arizona paleomagnetic results from 1090 ma diabase sheets of the southwestern u s a diabase province exposed in central arizona yield two distinct remanent magnetizations herein termed adn and adr in accordance with the findings of previous investigations magnetization adn is well defined and has an in situ mean direction of d 283 3 i 45 1 k 17 7 95 8 7 n 17 independent observations a mean pole after correction of paleomagnetic site means for a net 5 clockwise rotation of the colorado plateau and transition zone is located at 22 7 n 179 3 e k 21 9 a95 7 8 the second magnetization adr gives an in situ mean direction of d 161 1 i 87 5 k 22 2 95 19 9 n 4 independent observations with a poorly defined pole at 37 6 n 247 6 e k 6 5 a95 38 9 rock magnetic and alternating field and thermal demagnetization characteristics indicate the adn and adr magnetizations are both carried by low ti titanomagnetite both magnetizations are interpreted to be primary

north american apparent polar wander plate motion and left oblique convergence late jurassic early cretaceous orogenic consequences the north american apparent polar wander apw path indicates an episode of unusually rapid absolute northward motion of western north america between 150 and 135 ma during this time the northward component of absolute motion of points along the washington oregon california coast was in excess of 150 km m y and perhaps as high as 230 km m y we believe that such high absolute northward velocity for north america probably ensures that relative motions of oceanic plates and terranes influenced by them were to the south at this time the inception of rapid northward motion and left oblique convergence was abrupt and should be recorded in the geology of the western cordillera it is tempting to correlate this period of unusual pacific basin north american interaction with the nevadan orogeny in the klamath mountains as well as with left lateral strike slip structures such as the pine nut fault and bear mountains fault zone significant differences exist between north american plate motion recorded by the late jurassic cretaceous apw path and that predicted by a fixed hotspot model we believe that this discrepancy reflects uncertainty associated with pre late cretaceous hotspot tracks and poorly constrained relative plate motions during the cretaceous normal polarity superchron

paleomagnetism and rock magnetism of quaternary volcanic rocks and late paleozoic strata vc 1 core hole valles caldera new mexico with emphasis on remagnetization of late paleozoic strata paleomagnetic and rock magnetic data obtained from azimuthally unoriented core samples collected at approximately 1 to 3 m intervals of continental scientific drilling program core hole vc 1 have prompted reinterpretations of the quaternary volcanic stratigraphy intersected by the bore and have aided in evaluating the thermal regime within late paleozoic strata attending fluid circulation and mineral deposition during and after development of the toledo and valles calderas the results from quaternary units banco bonito obsidian i 35 4 a95 2 8 inclination only determinations n 33 battleship rock tuff d 359 6 i 42 4 a95 2 8 n 5 site means surface sites vc 1 rhyolite i 39 2 a95 12 8 n 7 upper vc 1 tuff i 37 2 a95 10 7 n 13 middle vc 1 tuff i 42 1 a95 2 1 n 39 south mountain rhyolite d 350 9 i 49 9 a95 3 4 n 10 one surface site are consistent with isotopic age data indicating that the entire moat volcanic sequence intersected is less than 650 kyr monitoring of natural remanent magnetization nrm intensity nrm directions directions of magnetizations isolated during progressive demagnetization median destructive forces and rock magnetization parameters has identified systematic variations within the thick banco bonito obsidian and vc 1 tuff units the permian abo formation pennsylvanian to earliest permian madera limestone and pennsylvanian sandia formation typically contain a moderate positive inclination magnetization component abo formation i 52 2 a95 7 4 n 16 madera limestone i 58 4 a95 2 8 n 105 sandia formation i 53 9 a95 4 8 n 21 when residing in magnetite it is usually unblocked in the laboratory by 350 c when carried by hematite it is unblocked by 550 c a moderate negative inclination e g madera and abo strata d 173 1 i 46 6 a95 5 5 n 47 samples assuming a north seeking positive inclination rm providing a useful means of core orientation oppositely directed magnetization is occasionally isolated and removed at higher laboratory unblocking temperatures up to 550 c in magnetite dominated and 600 c in hematite dominated lithologies where present magnetizations of shallower inclination e g madera and abo strata d 160 1 i 10 9 a95 4 4 n 28 are removed over higher ranges of unblocking temperatures and these are probably of late paleozoic age the moderate negative and positive inclination magnetizations are in all likelihood viscous partial thermoremanent magnetizations trms vptrms these were activated at moderate 300 c temperatures between 1 45 and 0 97 ma attending and following the main stages of toledo valles caldera development during the matuyama and at near present downhole temperatures 100 180 c during the brunhes and possibly within the last 10 kyr respectively importantly the preservation of negative inclination rm components in magnetite dominated lithologies of the madera limestone dictates that temperatures during the brunhes chron past 730 kyr did not exceed approximately 300 350 c the inferred temperatures for thermal activation of the vptrms based upon the middleton and schmidt thermal activation curves for both magnetite and hematite derived following walton agree with independent estimates of thermal activity using fluid inclusion and k ar isotopic age determination data during the quaternary the results further illustrate the potential usefulness of thermal activation theories for inferring temperatures of past geologic events especially when they can be applied to magnetizations of similar age residing in both magnetite and hematite and of paleomagnetic and rock magnetic studies in general in continuous coreholes in complex geologic environments

co seismic displacements folding and shortening structures along the chelungpu surface rupture zone occurred during the 1999 chi chi taiwan earthquake abstract a nearly 100 km long surface rupture zone called chelungpu surface rupture zone occurred mostly along the pre existing chelungpu fault on the northwestern side of taiwan accompanying the 1999 chi chi ms 7 6 earthquake the chelungpu surface rupture zone can be divided into four segments based on the characteristics of co seismic displacements geometry of the surface ruptures and geological structures these segments generally show a right step en echelon form and strike ne sw to n s and dip to the east with angles ranging from 50 to 85 the co seismic flexural slip folding structures commonly occurred in or near the surface rupture zone from a few meters to a few hundreds of meters in width which have an orientation in fold axes parallel or oblique to the surface rupture zone the displacements measured in the southern three segments are approximately 1 0 3 0 m horizontally and 2 0 4 0 m vertically the largest displacements were measured in the northern segment 11 1 m horizontally and 7 5 m vertically respectively the amount of co seismic horizontal shortening caused by flexural slip folding and reverse faulting in the surface rupture zone is generally less than 3 m it is evident that the co seismic displacements of the surface rupture zone are a quantitative surface indicator of the faulting process in the earthquake source fault the relations between the geometry and geomorphology of the surface rupture zone dips of the co seismic faulting planes and the striations on the main fault planes generated during the co seismic displacement show that the chelungpu surface rupture zone is a reverse fault zone with a large left lateral component

fault geometry and slip distribution of the 1999 chi chi taiwan earthquake imaged from inversion of gps data gps measurements of coseismic displacements from the 1999 chi chi taiwan earthquake are modeled using elastic dislocation theory we find that a single fault plane cannot fit the data but rather a curved fault surface consisting of multiple segments dipping 20 25 best fits the observations the model fault exhibits reverse and left lateral slip on a 75 km long n s trending segment and reverse and right lateral slip on a 25 km e w trending segment at the northern end of the rupture the 21 dipping e w segment is inconsistent with previous interpretations of high angle tear faulting

plate motions crustal and lithospheric mobility and paleomagnetism prospective viewpoint the shift in viewpoint from a static to a mobilistic solid earth as brought about by paleomagnetism opened many new directions of research that continue to be exciting today and hold prospects for an exciting future here five topics of current research on earth s mobilistic surface are reviewed and their future directions discussed first dividing earth s solid surface into two domains 1 nearly rigid plate interiors and 2 plate boundaries some of them very wide leads to a review and discussion of the kinematics of wide plate boundaries many more adjustable parameters will be needed to describe the complex kinematics of plate boundaries which cover 15 of earth surface than the few dozen parameters needed to describe the kinematics of plate interiors which cover 85 of earth s surface second the question of the degree of rigidity of plate interiors is discussed the integral of the velocity gradient across plates interiors is at least a few hundredths or tenths of millimeters per year and in several well determined cases less than 2 or 3 mm yr future investigations will no doubt be aimed at narrowing these limits and to expanding the area of earth s surface over which limits are known third space geodetic data mainly from very long baseline inter ferometry satellite laser ranging and the global positioning system have demonstrated a remarkable similarity of velocities of stable plate interiors averaged over a few to a dozen years with plate velocities averaged over a few million years already however significant differences of a few millimeters per year are emerging and the tectonic and dynamic significance of these differences need to be evaluated fourth despite more than two decades of investigation the question of how fast hotspots move relative to one another is still a contentious issue one group of researchers maintains that maximum speeds are 3 mm yr whereas another maintains that speeds are 10 20 mm yr or more the differences cannot be reconciled until the uncertainties in plate reconstructions relative to hotspots are systematically and properly incorporated into analyses of hotspot motion fifth paleomagnetists have estimated true polar wander over the past 200 myr by equating it with the apparent polar wander of hotspots however the possible motion among hotspots and the neglect of the uncertainties in reconstructions relative to the hotspots have left these analyses unconvincing further progress requires the incorporation of these uncertainties a possible exception is the apparent polar wander of the hotspots over the past 10 20 myr or less which requires only small adjustments for plate motion and has presumably smaller uncertainties due to plate reconstructions the cause of this apparently significant geologically recent shift of the pole is poorly understood compared with the rate and direction of the secular shift of the pole observed this century from the international latitude service and for 1976 to 1994 from space geodetic data the paleomagnetically observed shift is unlikely to be due to the removal of northern hemisphere midlatitude ice sheets which is the commonly accepted explanation for the shift observed over the past century i speculate instead that the apparently rapid uplift of the tibetan plateau sometime during the past 10 20 myr is at least partly responsible for the geologically recent shift of the pole finally i conclude that the theory of plate tectonics has provided a framework that leads naturally to further quantification of the kinematics and deformation of earth s solid surface chiefly because of the key assumption of the rigidity of plate interiors which permits specific predictions to be made this revolution in quantification still has far to go and holds exciting prospects for future tectonic studies

non double couple earthquakes 1 theory historically most quantitative seismological analyses have been based on the assumption that earth quakes are caused by shear faulting for which the equiv alent force system in an isotropic medium is a pair of force couples with no net torque a double couple or dc observations of increasing quality and coverage however now resolve departures from the dc model for many earthquakes and find some earthquakes especially in volcanic and geothermal areas that have strongly non dc mechanisms understanding non dc earth quakes is important both for studying the process of faulting in detail and for identifying nonshear faulting processes that apparently occur in some earthquakes this paper summarizes the theory of moment tensor expansions of equivalent force systems and analyzes many possible physical non dc earthquake processes contrary to long standing assumption sources within the earth can sometimes have net force and torque components described by first rank and asymmetric sec ond rank moment tensors which must be included in analyses of landslides and some volcanic phenomena non dc processes that lead to conventional symmetric second rank moment tensors include geometrically complex shear faulting tensile faulting shear faulting in an anisotropic medium shear faulting in a heteroge neous region e g near an interface and polymorphic phase transformations undoubtedly many non dc earthquake processes remain to be discovered progress will be facilitated by experimental studies that use wave amplitudes amplitude ratios and complete waveforms in addition to wave polarities and thus avoid arbitrary assumptions such as the absence of volume changes or the temporal similarity of different moment tensor com ponents

fracture network of the ferron sandstone member of the mancos shale east central utah usa abstract the fracture network at the outcrop of the ferron sandstone member of the mancos shale was studied to gain an understanding of the tectonic history of the region and to contribute data to studies of gas and water transmissivity related to the occurrence and production of coal bed methane about 1900 fracture readings were made at 40 coal outcrops and 62 sandstone outcrops in the area from willow springs wash in the south to farnham dome in the north of the study area in east central utah two sets of regional vertical to nearly vertical systematic face cleats were identified in ferron coals a northwest striking set trends at a mean azimuth of 321 and a northeast striking set has a mean azimuth of 55 cleats were observed in all coal outcrops examined and are closely spaced and commonly coated with thin films of iron oxide two sets of regional systematic joint sets in sandstone were also identified and have mean azimuths of 321 and 34 the joints of each set are planar long and extend vertically to nearly vertically through multiple beds the northeast striking set is more prevalent than the northwest striking set in some places joints of the northeast striking set occur in closely spaced clusters or joint zones flanked by unjointed rock both sets are mineralized with iron oxide and calcite and the northwest striking set is commonly tightly cemented which allowed the northeast striking set to propagate across it all cleats and joints of these sets are interpreted as opening mode mode i fractures abutting relations indicate that the northwest striking cleats and joints formed first and were later overprinted by the northeast striking cleats and joints burial curves constructed for the ferron indicate rapid initial burial after deposition the ferron reached a depth of 3000 ft 1000 m within 5 2 million years m y and this is considered a minimum depth and time for development of cleats and joints the sevier orogeny produced southeast directed compressional stress at this time and is thought to be the likely mechanism for the northwest striking systematic cleats and joints the onset of the laramide orogeny occurred at about 75 ma within 13 7 m y of burial and is thought to be the probable mechanism for development of the northeast striking systematic cleats and joints uplift of the ferron in the late tertiary contributed to development of butt cleats and secondary cross joints and probably enhanced previously formed fracture sets using a study of the younger blackhawk formation as an analogy the fracture pattern of the ferron in the subsurface is probably similar to that at the surface at least as far west as the paradise fault and joe s valley graben farther to the west on the wasatch plateau the orientations of ferron fractures may diverge from those measured at the outcrop

slip history and dynamic implications of the 1999 chi chi taiwan earthquake we investigate the rupture process of the 1999 chi chi taiwan earthquake using extensive near source observations including three component velocity waveforms at 36 strong motion stations and 119 gps measurements a three plane fault geometry derived from our previous inversion using only static data ji et al 2001 is applied the slip amplitude rake angle rupture initiation time and risetime function are inverted simultaneously with a recently developed finite fault inverse method that combines a wavelet transform approach with a simulated annealing algorithm ji et al 2002b the inversion results are validated by the forward prediction of an independent data set the teleseismic p and sh ground velocities with notable agreement the results show that the total seismic moment release of this earthquake is 2 7 10 20 n m and that most of the slip occurred in a triangular shaped asperity involving two fault segments which is consistent with our previous static inversion the rupture front propagates with an average rupture velocity of 2 0 km s 1 and the average slip duration risetime is 7 2 s several interesting observations related to the temporal evolution of the chi chi earthquake are also investigated including 1 the strong effect of the sinuous fault plane of the chelungpu fault on spatial and temporal variations in slip history 2 the intersection of fault 1 and fault 2 not being a strong impediment to the rupture propagation and 3 the observation that the peak slip velocity near the surface is in general higher than on the deeper portion of the fault plane as predicted by dynamic modeling