active flexural slip faulting a study from the pamir tian shan convergent zone nw china the flexural slip fault fsf a type of secondary fault generated by bed parallel slip occurs commonly and plays an important role in accommodating fold growth although the kinematics and mechanics of fsfs are well studied relatively few field observations or geometric models explore its geomorphic expression in the pamir tian shan convergent zone nw china suites of well preserved fsf scarps displace fluvial terraces in the mingyaole and wulagen folds integrating interpretations of google earth images detailed geologic and geomorphic mapping and differential gps measurements of terrace surfaces we summarize geomorphic features that typify these faults and create kinematic models of active flexural slip faulting our study indicates the following i fsf scarps commonly occur near synclinal hinges irrespective of whether a the dip direction of beds on either side of the hinge is unidirectional or in opposite directions b the hinge is migrating or fixed or c the hinge shape is narrow and angular or wide and curved ii active fsfs are likely to produce higher scarps on steeper beds whereas lower or no topographic scarps typify gentler beds iii tilt angles of the terrace surface displaced above fsfs progressively decrease farther away from the hinge with abrupt changes in slope coinciding with fsf scarps the changes in tilt angle and scarp height have a predictable geometric relationship iv active fsfs can accommodate a significant fraction of total slip and play a significant role in folding deformation v active fsfs may be used to assess seismic hazards associated with active folds and associated blind thrusts

alternating field characteristics of pseudo single domain 2 14 μm and multidomain magnetite abstract we report normalized af demagnetization curves of anhysteretic remanences arm s produced by 1 10 and 40 oe steady fields and of saturation isothermal remanence irm s in a suite of dispersed unannealed magnetite powders with median sizes of 2 4 6 10 and 14 m pseudo single domain or psd size range and 100 m multidomain or md size interpreted in the light of the domain structure test first proposed by lowrie and fuller 12 the relative stability trend of curves for the 2 m sample is of single domain sd type the 1 oe arm being most resistant to demagnetization followed by the 10 oe and 40 oe arm s and irm s for the 100 m sample the trend is exactly reversed and is of md type in the 4 14 m samples hitherto undescribed transitional trends between sd type and md type occur at 6 m 1 oe 10 oe and 40 oe arm s preserve an sd type trend but for all af s 75 oe irm s is more resistant than any of these remanences at 10 m this trend is unmistakable and only at 14 m do the 1 oe 10 oe and 40 oe arm curves merge we conclude 1 that the lowrie fuller test distinguishes between small md grains enhanced by psd remanence and large md grains lacking psd remanence rather than between sd and md structures per se and 2 that in the psd transition region from 6 to 14 m in magnetite irm s changes over to md type relative stability around 6 m whereas 10 oe and 40 oe arm s achieve an md type trend around 14 m in accord with the predicted field dependence of the psd threshold size our theoretical interpretation assumes that the intrinsic internal field coercive force spectra of weak field and strong field remanences are identical but that the observed external field spectrum is shifted to lower fields as a result of the internal demagnetizing field nj r of the remanence j r the effect is slight for weak field j r s but substantial for irm s since all coercivities high as well as low are shifted the result of the lowrie fuller test is determined simply by the shape of the intrinsic coercivity spectrum or the corresponding af demagnetization curve depending on the model of self demagnetization used either subexponential or sublinear af decay curves of weak field remanence will automatically lead to an md type trend whereas by either model the decay curves that characterize sd and psd remanences decaying slowly initially and then more rapidly will always produce and sd type trend

the ms 7 3 1992 suusamyr kyrgyzstan earthquake in the tien shan 2 aftershock focal mechanisms and surface deformation abstract the m s 7 3 suusamyr earthquake of 19 august 1992 was a large thrust type earthquake in the northern tien shan of kyrgyzstan an intracontinental mountain belt in central asia focal mechanisms of both the mainshock and 13 moderate sized aftershocks are consistent with a seismotectonic setting that is dominated by nnw sse horizontal compression which manifests itself in reverse faulting along roughly east west trending steeply dipping fault planes the earthquake produced surface faulting at two widely separated locations along two different fault segments the total length of which was less than 4 km detailed measurements at one of the locations indicate a net slip of at least 4 2 0 4 m a wide variety of secondary surface effects was observed in the macroseismic zone that extended over an area of 2500 km 2 the lack of extensive surface rupture from such a large event can be explained by a combination of 1 redistribution of the net slip at depth among the secondary splays that branches out from the main fault near the surface and 2 variable surface slip over the fault length the active deformation associated with this event documents a structural style that involves steeply dipping reverse faults extending from mid crustal depths to the surface at nearly uniform dip accommodating crustal shortening in the interior of the tien shan

earthquakes generated from bedding plane parallel reverse faults above an active wedge thrust seattle fault zone a key question in earthquake hazard analysis is whether individual faults within fault zones represent independent seismic sources for the seattle fault zone an upper plate structure within the cascadia convergent margin evaluating seismic hazard requires understanding how north side up bedding plane reverse faults which generate late holocene fault scarps interact with the north vergent master ramp thrust and overlying backthrust of the fault zone a regional uplift at a d 900 930 involved an earthquake that nucleated at depth and included slip on both the master ramp thrust and the backthrust this earthquake also included slip on some of the 6 km deep north side up bedding plane reverse faults at locales where the north side up reverse faults intersect the puget sound coast an earthquake a few centuries earlier than the a d 900 930 regional uplift only uplifted areas within hundreds of meters north of the reverse faults we infer that the bedding plane reverse faults are seismogenic because shore platforms near the reverse faults have been abruptly uplifted during earthquakes when other shorelines in the seattle fault zone were unaffected faults of the seattle fault zone therefore can both produce regional uplift earthquakes with or without surface displacement on the reverse faults and produce earthquakes that rupture the bedding plane reverse faults causing fault scarps and uplift localized to hundreds of meters north of these faults this latter type of earthquake has occurred at least twice and perhaps three times in the late holocene and all these earthquakes preceded the regional coseismic uplift of a d 900 930 to account for the paleoseismic observations we propose that the seattle fault zone is a wedge thrust with the leading edge being a fault bend wedge thrust fold the active axial surface of the wedge thrust fold is pinned at the tip of the wedge and a steeply north dipping sequence of tertiary sediment forms the south limb of the wedge thrust fold some of these steeply north dipping bedding plane surfaces are seismogenic reverse faults that produce scarps earthquakes on the wedge thrust produce the regional coseismic uplift events and earthquakes within the faultbend fold cause the local uplift earthquakes thus bedding plane faults can rupture during earthquakes when the wedge thrust does not rupture but instead continues to accumulate seismic energy

the elkhorn mountains revisited new data for the late cretaceous paleomagnetic field of north america the late cretaceous portion of north america s apparent polar wander apw path is poorly known due to a dearth of suitable post 85 ma reference poles a paleomagnetic re investigation of elkhorn mountains volcanics 80 ma was initiated to fill this void and to determine the degree to which the onset of an apparent rapid episode of apw during this time interval correlates with known late cretaceous tectonic events a total of 249 paleomagnetic samples 30 sites were collected from three separate areas within the elkhorn mountains using both alternating field and thermal demagnetization a characteristic direction of magnetization was isolate at 15 sites a95 15 which passes a fold test at the 95 confidence level the mean pole calculated from site virtual geomagnetic poles is located at 80 3 n 189 5 e n 15 a95 9 6 k 16 8 interestingly the elkhorn mountains pole is not concordant with the cretaceous stillstand poles but is nearly coincident with the paleocene reference pole and the recently published pole for the adel mountains dated at 76 ma these new data suggest the onset of apw which brought the pole from its cretaceous position to near its early tertiary position occurred prior to 80 ma and the transition was exceedingly rapid the onset of this rapid apw now appears to coincide more closely in time with many of the significant late cretaceous tectonic events e g the laramide orogeny however since the elkhorn mountains volcanics are floored by a major east directed thrust and have undergone crustal scale deformation the data should be viewed with caution until more results are available for this time interval

active deformation of the pacific northwest continental margin tilted and uplifted marine terraces in southern oregon show progressive landward tilting of the coastal ranges at about 5 16 10 8 rad yr 1 for the last 0 25 m y tide gauges in washington and british columbia and ten resurveyed leveling lines running inland from the coast indicate contemporary landward down to the east tilt rates of about 1 12 10 8 rad yr 1 averaged over periods of from 10 to 50 years the leveling lines traverse and the terraces cut across dipping cenozoic strata pleistocene dips to 3 mio pliocene dips to 30 and eocene dips to 60 southern oregon from cape blanco to the siletz river shows geodetic or terrace tilting in the same direction as the underlying stratal dips hence present day deformation continues past deformation of the coastal ranges and is most likely related to active subduction of the juan de fuca plate the steep stratal dips lack of major active faults and historic earthquakes and presence of very young bedding plane faults suggest that much of the onshore deformation and shortening within the overlying north american plate is taken up by folding rather than thrust faulting present shortening rates across north trending folds near the coast are between 0 02 and 1 9 10 7 yr 1 the rate of shortening decreases rapidly eastwards from the juan de fuca north american plate boundary a total of about 25 mm yr 1 of permanent shortening could be occurring within the north american plate most of it in the westernmost 40 km the landward tilt and shortening rates are similar to those above many other subduction zones that have experienced great thrust earthquakes while a high strain rate measured near seattle washington has been interpreted as elastic strain accumulation before a thrust earthquake the low level of historic seismicity and the similarity of short and long term deformation rates suggest alternatively that the subduction beneath washington is aseismic the issue has considerable implications for seismic hazard evaluation in the pacific northwest and could be resolved by a search for the effects or lack of effects of prehistoric great earthquakes

paleomagnetic investigations in igneous metamorphic rock units in eastern new england that rising orogenic belt such as the l hi would appear to be later than all rotational tectonic disturprovide a better opportunity for finding such methodol bances in an rep0 schutts et al 1976 we ogy because of the much more detailed knowledge of outlined some successes in meeting this first objective superimposed events there as compared with the case of particularly in the use rock systems that the eroded equivalent in a bri shield area h pervasive alterations since the first report we have l hi about which there has been a extended the investigation over rock systems from wealth of detailed geological and geochronological massachusetts to new brunswicky and from mafic studies was considered ideal metamorphic overprints and rocks to potassic are pervasive and are known regionally through k a granophyres in general the results have been successdating i events have been dated by various ful so that we now believe that it is possible to meet the so that in a number of specific cases their ages principal objective if the investigator follows prescribed are known with some degree of confidence geological procedures

magma sources for mesozoic anorogenic granites of the white mountain magma series new england usa the magma sources for granitic intrusions related to the mesozoic white mountain magma series in northern new england usa are addressed relying principally upon nd isotopes many of these anorogenic complexes lack significant volumes of exposed mafic lithologies and have been suspected of representing crustal melts sm nd and rb sr isotope systematics are used to evaluate magma sources for 18 felsic plutons with ages ranging from about 120 to 230 ma the possibility of crustal sources is further examined with analyses of representative older crust including paleozoic granitoids which serve as probes of the lower crust in the region multiple samples from two representative intrusions are used to address intrapluton initial isotopic heterogeneities and document significant yet restricted variations 1 in end overall mesozoic granite plutons range in end t from 4 2 to 2 3 with most 2 to 0 and in initial 87sr 86sr from 0 7031 to 0 709 the isotopic variations are roughly inversely correlated but are not obviously related to geologic geographic or age differences older igneous and metamorphic crust of the region has much lower nd isotope ratios with the most radiogenic paleozoic granitoid at end 180 ma of 2 8 these data suggest mid proterozoic separation of the crust in central northern new england moreover the bulk of the mesozoic granites cannot be explained as crustal melts but must have large mantle components the ranges of nd and sr isotopes are attributed to incorporation of crust by magmas derived from midly depleted mantle sources crustal input may reflect either magma mixing of crustal and mantle melts or crustal assimilation which is the favored interpretation the results indicate production of anorogenic granites from mantle derived mafic magmas

fault slip rates and earthquake histories for active faults in southern california abstract within the context of the historical record of seismicity we review the geological data bearing on the quaternary slip rates and paleoearthquake histories of active faults in southern california

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

mid cretaceous paleomagnetic reference field for north america restudy of 100 ma intrusive rocks from arkansas we have obtained new paleomagnetic data from three middle cretaceous alkalic intrusive complexes in central arkansas magnet cove and potash sulfur springs intrusions about 100 ma 20 sites and granite mountain stock about 88 ma 6 sites out of 254 specimens measured 147 from 20 sites were accepted the remainders have scattered magnetization directions which experiments indicate were caused by lightning occurring during frequent electrical storms in this region the geologic evidence indicates that the intrusions have not been tilted since emplacement the directions of magnetization calculated with respect to present day horizontal yield a mean paleopole at 192 5 e 74 1 n n 20 k 34 a95 5 7 this does not differ at 95 confidence from that determined previously by scharon and hsu but the error is diminished by half the angular dispersion sd of site virtual geomagnetic poles vgps is 13 6 similar to the value of sd about 15 predicted by current paleosecular variation models the revised arkansas paleopole is combined with four other cratonic north american paleopoles of mid cretaceous age to yield a revised middle cretaceous reference pole at 196 e 71 n a95 4 9 the mean of nine paleopoles derived from overprints in rock units from the craton and craton margin which range in age from proterozoic to jura cretaceous is 202 e 68 n a95 8 2 this is not different from the reference paleopole indicating that overprinting was very widespread across north america in mid cretaceous time

linear and planar structure in ordered multivariate data as applied to progressive demagnetization of palaeomagnetic remanence summary consider a sequence of say 10 to 20 vector observations in threedimensional space it is suspected that a few subsets of consecutive observations are made up of collinear points the purpose of this paper is to construct a statistically based algorithm to find such linear segments and to assess their accuracy a similar assessment is made for coplanar sets of points this algorithm is applied here to palaeomagnetic data and is claimed to be superior to previous methods of palaeomagnetic analysis in terms of completeness and balance of analysis treatment of measurement errors and other sources of scatter criteria for identification of linear and planar sets of points and statistical rigour stability spectra with statistically based confidence limits are obtained as a by product 1 palaeomagnetic analysis background to the geometrical problem the natural remanent magnetization nrm in rocks is known in general to have originated in one or more relatively short time intervals during their formation and subsequent history i e during rock forming or metamorphic events during which nrm is frozen in stabilized by falling temperature grain growth etc the nrm acquired during each such event is a single vector magnetization parallel to the then prevailing geomagnetic field and is called a component of nrm rocks which have suffered more than one event of this kind are therefore liable to carry multi component nrm in the laboratory these components can be identified by demagnetizing the rock step by step usually by thermal alternating field or chemical demagnetization which involves the analysis of the distribution of blocking rzmperature coercivity and chemical leaching respectively resistance to these treatments is called stability of remanence and the range of treatment over which a particular component is removed is called the stability range of the component the various discrete

relations between submarine fissures internal breccias and mass flows during triassic and earlier rifting periods deep fissures in shallow water limestones filled with deep water sediments are described from many places especially in the triassic and jurassic of the tethys and in the middle to upper devonian of the variscian mountains mass flows occur in the same formations and areas on the island of hydra greece it was possible to show that both phenomena are linked by internal breccias of well fitted limestone fragments they are indicative of slope or near slope positions and are very probably caused by flexure like deformations of shallow burial limestones seismic shocks producing overpressure may be instrumental in the disintegration of the semi lithified limestones thus providing the material for mass flows these events are associated with a tectonical subsidence as shown by deep water matrix in the brecciated shallow water limestones they occur in areas and periods of submarine rifting especially in late devonian and triassic time at least four triassic brecciation events can be traced from greece to the alps and correspond presumably to major transgressions in the cratonic basin of middle europe

paleomagnetism of the roskruge and gringo gulch volcanics southeast arizona paleomagnetic data were obtained from nine cooling units of the late cretaceous 72 ma roskruge volcanics and from 25 flows in the lower paleocene gringo gulch volcanics both from southeastern arizona alternating field demagnetization successfully erased the infrequent secondary components of magnetization the paleomagnetic pole position obtained from the roskruge volcanics is at 73 6 n 176 0 e with dp 6 2 and dm 8 8 the gringo gulch volcanics pole position is at 77 0 n 201 0 e with dp 1 2 and dm 1 7 in conjunction with other recently published paleocene and eocene paleomagnetic poles these data provide details of north american apparent polar wander during the laramide orogeny 80 to 40 ma an episode of rapid apparent polar wander from the cretaceous pole position in the bering strait to the eocene pole position near the present rotation axis occurred during that interval of time the initiation of this episode of apparent polar wander appears to be coincident with the major plate reorganizations which occurred at the onset of the laramide orogeny

basanite glaciovolcanism at llangorse mountain northern british columbia canada the llangorse volcanic field is located in northwest british columbia canada and comprises erosional remnants of miocene to holocene volcanic edifices lava flows or dykes the focus of this study is a single overthickened 100 m thick valley filling lava flow that is middle pleistocene in age and located immediately south of llangorse mountain the lava flow is basanitic in composition and contains mantle derived peridotite xeno liths the lava directly overlies a sequence of poorly sorted crudely bedded volcaniclastic debris flow sediments the debris flow deposits contain a diverse suite of clast types including angular clasts of basanite lava blocks of peridotite coated by basanite and rounded boulders of granodiorite many of the basanite clasts have been palagonitized the presence and abundance of clasts of vesicular to scoriaceous palagonitized basanite and peridotite suggest that the debris flows are syngenetic to the overlying lava flow and sampled the same volcanic vent during the early stages of eruption they may represent lahars or outburst floods related to melting of a snow pack or ice cap during the eruption the debris flows were water saturated when deposited the rapid subse quent emplacement of a thick basanite flow over the sediments heated pore fluids to at least 80 100 c causing in situ palagonitization of glassy basanite clasts within the sediments the over thickened nature of the llangorse mountain lavas suggests ponding of the lava against a down stream barrier the distribution of similar aged glaciovolcanic features in the cordillera suggests the possibility that the barrier was a lower elevation valley wide ice sheet

paleomagnetism of middle late triassic plutons in southern maine fang wu and van der voo r 1988 paleomagnetism of middle late triassic plutons in southern maine tecionophysics 156 51 58 a paleomagnetic study of two small syenitic to alkali granitic plutons abbott and agamenticus southern maine yields paleopoles at 48o n 92 e and 48o n 99 o e with k ar ages of 221 f 8 and 228 5 ma respectively a third pluton the litchfield was much more weathered and did not contain stable magnetizations dual polarity magnetizations a dike contact test and excellent statistical parameters obtained after detailed thermal and alternating field demagnetization support our claim that the magnetizations and paleopoles are representative of the middle to late triassic paleomagnetic field however the two paleopoles fall about 10 to the southwest of previously published triassic apparent polar wander path segments for cratonic north america unless this small deviation can be ascribed to post triassic tilt of the plutons this requires a modification of the polar wander path to bring the poles into agreement a tilt correction of some 15 20 needs to be applied about a strike which is perpendicular to the area s geologic trends and the main coast line which appears to be highly unlikely given the stabilized tectonic setting of this area in post triassic time a small but significant revision of the north american cratonic apparent polar wander path may therefore be suggested by these results but this needs further corroboration the more southerly paleolatitudes indicated by these triassic results for north america would imply lesser displacements than previously proposed on the basis of triassic paleomagnetism for exotic terranes in the western cordillera

patterns of historical earthquake rupture in the iranian plateau the iranian plateau accommodates the 35 mrn yr convergence rate be tween the eurasian and arabian plates by strike slip and reverse faults with relatively low slip rates in a zone 1000 km across although these faults have only locally been the subject of paleoseismological studies a rich historical and archeological record spans several thousand years long enough to establish recurrence intervals of 1000 to 5000 yr on individual fault segments several clusters of earthquakes provide evidence of interaction among reverse and strike slip faults probably due to adjacent faults being loaded by individual earthquakes the dasht e bayaz sequence of 1936 to 1997 includes earthquakes on left lateral right lateral and reverse faults the neyshabur sequence of four earthquakes between 1209 and 1405 respected the seg ment boundary between the neyshabur and binalud reverse fault systems the two pairs of earthquakes may have ruptured different faults in each segment similar to the 1971 and 1994 san fernando california earthquakes the 1978 tabas reverse fault earthquake was preceded by the 1968 ferdows earthquake part of the dasht e bayaz sequence the north tabriz fault system ruptured from southeast to north west in three earthquakes from 1721 to 1786 a previous cluster may have struck this region in 855 to 958 the mosha fault north of tehran ruptured in three earthquakes in 958 1665 and 1830 five large earthquakes struck the tehran region from 743 to 1177 but only two that large have struck the area since 1177 other earthquakes occurred in pairs in the talesh mountains near the caspian sea 1863 1896 the iran turkey border 1840 1843 and the nayband gowk fault system both 1981 other historical events did not occur as parts of sequences the historic seismic moment release in iran accounts for only a small part of the plate convergence rate which may be due to aseismic slip or to the iranian historical record long as it is being too short to sample long term deformation across the plateau no historic earthquakes of m 8 have struck iran however several long straight strike slip faults doruneh west neh east neh and nayband have not sustained large historical earthquakes raising the possibility that these long faults could produce earthquakes of m 8 thereby removing at least part of the apparent slip deficit an increased understanding of iran s seismic hazard could be obtained by an extensive paleoseismology program and space geodetic arrays supplementing the abundant historical and archaeological record

earthquakes quaternary faults and seismic hazard in california data describing the locations slip rates and lengths of quaternary faults are the primary basis in this work for constructing maps that characterize seismic hazard in california the expected seismic moment m0e and the strength of ground shaking resulting from the entire rupture of each mapped fault or fault segment are estimated using empirical relations between seismic moment m0 rupture length source to site distance and strong ground motions assuming a fault model whereby the repeat time t of earthquakes on each fault equals m0e m0g where the moment rate m0g is proportional to fault slip rate it is observed that the moment frequency distribution of earthquakes predicted from the geologic data agrees well with the distribution determined from a 150 year historical record the agreement is consistent with the argument that the geologic record of quaternary fault offsets contains information sufficient to predict the average spatial and size distribution of earthquakes through time in california the estimates of t for each fault are the foundation for constructing maps that depict the average return period of 0 1g peak horizontal ground accelerations and the horizontal components of peak acceleration peak velocity and the pseudovelocity response at 1 period and 5 damping expected to occur at the level of 0 1 probability during a 50 year period of time a map is also formulated to show the probability that 0 1g horizontal ground accelerations will occur during the next 50 years the maps serve to illustrate the potential value of quaternary fault studies for assessing seismic hazard interpretation of available slip rates indicates that the largest and most frequent occurrence of potentially destructive strong ground motions are associated principally with the san andreas san jacinto calaveras hayward and ventura basin fault zones other regions of similarly high hazard may yet remain unrecognized this inadequacy results primarily from an incomplete data set numerous faults for example are mapped along the coastal region of northern california and within the modoc plateau but relatively few studies relating to fault slip rate are reported a similar problem exists for other stretches of coastal california where marine reflection studies provide evidence of active faulting offshore yet yield little or no information of fault slip rate geological and geophysical field studies can work to remove these deficiencies a concerted effort to locate and define rates of activity on all faults in california is the most promising means to further quantify present levels of seismic hazard in california

a new late cretaceous paleomagnetic pole from the adel mountains west central montana north america s apparent polar wander path has been poorly defined between the mid cretaceous and paleocene reference pole positions existing data allowed 13 of apparent polar motion over about 22 m y 87 65 ma roughly coinciding with the beginning of laramide deformation 80 ma we report on a paleomagnetic study of the adel mountain volcanic rocks to refine the north american apparent polar wander path for this interval the shonkinite rocks of the adel mountain volcanic field are on the eastern edge of the cretaceous paleocene fold and thrust belt some of these structures disturb the western edge of the volcanic pile we obtained two new k ar dates from the adel rocks one date on biotite from a shonkinite dike that crosscuts most of the volcanic rocks is 71 2 2 7 ma the other a whole rock date from a flow deep in the volcanic pile is 81 1 3 5 ma we collected six to nine paleomagnetic samples from each of 34 sites in roadcuts and natural outcrops of flows dikes and laccoliths positive fold and conglomerate tests along with alternating field and thermal demagnetization indicate that our characteristic remanent directions are primary magnetizations acquired before late cretaceous to paleocene thrust belt deformation averaging the virtual geomagnetic poles from 26 reliable sites all of normal polarity yields a paleopole at 82 2 n 209 9 e 95 6 80 k 18 38 this pole is concordant with the paleocene reference pole 82 0 n 170 2 e 95 3 5 k 18 6 diehl et al 1983 and is 11 6 from the globerman and irving 1988 mid cretaceous pole at 71 n 196 e the youngest information in the cretaceous stillstand pole is from the niobrara formation shive and frerichs 1974 at about 85 89 ma if we take the average age of the adel mountain volcanics to be 76 ma then 12 of apparent polar motion occurred between 87 ma and 76 ma thus rapid apparent polar motion correlates well with the onset of laramide deformation

40ar 39ar ages for plutons of the monteregian hills quebec evidence for a single episode of cretaceous magmatism 40ar 39ar data for biotite and amphibole from six plutonic complexes of the monteregian petrographic province in quebec canada are presented generally uncomplicated and concordant incremental heating spectra are observed although there is evidence for minor excess 40ar at several localities biotites and amphiboles are concordant consistent with rapid cooling of the high level complexes ages for different phases of individual complexes are analytically indistinguishable and suggest that the various principal lithologies are cogenetic the 40ar 39ar ages of six monteregian complexes st hilaire rougemont johnson shefford brome megantic all fall within the very restricted range of 124 1 m y it is suggested that all of the monteregian plutons with the exception of oka were formed within a short interval 1 2 m y during the cretaceous consistent with paleomagmetic results the new ages along with geochemical characteristics support derivation of the alkaline magmas by a fixed mantle hot spot which also produced broadly similar complexes in new england and the chain of new england seamounts

paleomagnetism of 122 ma plutons in new england and the mid cretaceous paleomagnetic field in north america true polar wander or large scale differential mantle motion a paleomagnetic study of cretaceous white mountains plutonic complexes in new hampshire and vermont yields high unblocking temperature dual polarity magnetizations in different types of igneous rocks the resulting pole position for three plutons 71 9 n 187 4 e a95 6 9 age 122 5 ma agrees with previously published mid cretaceous poles for north america which together give a mid cretaceous standstill reference pole slightly revised from globerman and irving 1988 at 71 2 n 194 1 e a95 3 7 n 5 studies we argue on the basis of the wide geographic distribution of these studies the variety in tectonic settings and rock types positive reversal tests and an overall reversal pattern consistent with geomagnetic polarity time scales that this mean pole represents the north american mid cretaceous reference field for nominally 36 m y 124 to 88 ma the standstill pole limits to within 4 the motion of the north american plate relative to the earth s spin axis during the same mid cretaceous interval the new england hotspot track 124 ma monteregian hills 122 5 ma cretaceous white mountains and 103 to 84 ma new england seamounts requires 11 4 of north poleward motion of north america in direct conflict with the paleomagnetic standstill a similar 13 discrepancy is independently demonstrated between the spin axis and the tristan da cunha hotspot track on the african plate during the mid cretaceous interval the hotspot spin axis discrepancies ended by 90 ma when it is shown that both atlantic hotspots agree with north american and african dipole paleolatitudes and present day locations nondipole fields are an unlikely explanation of the uniform motion of these two widely separated hotspots with respect to the spin axis leaving as possible interpretations true polar wander and large scale but differential mantle motion the southerly motion of the mid cretaceous louisville hotspot relative to the spin axis is ostensively at odds with what would be predicted under the true polar wander interpretation and points to differential mantle kinematics the motions of the three widely separated mid cretaceous hotspots with respect to the spin axis may be related to the recently proposed increase in global oceanic lithosphere production rates which gave rise to the mid cretaceous superplume

late quaternary tectonic deformation in the cape arago bandon region of coastal oregon as deduced from wave cut platforms the cape arago region of south central oregon sits on the upper plate of the cascadia subduction zone about 80 km east of the base of the continental slope the style of late pleistocene deformation along the cascadia forearc near cape arago is well expressed by the altitudinal variation of a set of five uplifted wave cut platforms these platforms record open folding with a half wavelength of about 6 7 km as well as late quaternary offset on flexural slip reverse faults that parallel bedding in the underlying bedrock the folds have produced both landward and seaward tilts to the uplifted wave cut platforms because the folds cut obliquely across the coastline the magnitude of coastal uplift is variable in the case of the lowest 80 ka wave cut platform this variable uplift has resulted in coastal deformation ranging from subsidence to a maximum uplift of 0 8 m kyr quantitative analysis of the mechanism of flexural slip folding for the south slough syncline near cape arago indicates that the late quaternary strain rate has decreased in the last 200 kyr evidence of past great subduction style earthquakes such as regional uplift and regional landward tilting of wave cut platforms or regional submergence of coastlands is lacking inmore the cape arago region instead the deformational style is controlled by folding though localized folding is dominant the occurrence of great subduction style earthquakes is not precluded because localized folding could occur concurrently with regional coseismic deformation less

active thrusting and folding in the qilian shan and decoupling between upper crust and mantle in northeastern tibet fieldwork south of the city of gaotai gansu province china shows that active shortening of surface sediments in the foothills of the yumu shan a large fore mountain of the qilian shan at the northeastern edge of tibet involves both overthrusting and flexural slip folding north of this mountain we found and mapped a prominent north facing thrust scarp that offsets a holocene fan sloping gently 3 4 to the north part of this scarp appears to be related to the m 7 5 180 a d earthquake that may have led to the demise of the han dynasty city of luo tuo chen in the hexi corridor a set of 10 100 150 m long profiles measured across this scarp 3 2 m high on the average can be made to fit the diffusion degraded morphology of a surface break related to the 180 a d event using a value of about 3 3 m 2 10 3 yr for the mass diffusivity of fanglomerates in this part of gansu province smaller mountain facing scarps on a terrace capped foothill result from bedding slip concurrent with active folding of underlying steeply northdipping plioquaternary sandstone and conglomerate beds holocene uplift rates along the yumu shan which is only one of the qilian shan ranges are estimated to be between 0 4 and 1 9 mm yr which implies that much of the mountain formed in the quaternary the periclinal structure of the plioquaternary envelope under which the paleozoic core of the yumu shan plunges towards the west suggests that the whole 3200 m high mountain is a basement ramp anticline mountains striking parallel to the yumu shan with similar structure and comparable or greater sizes north and south of the hexi corridor probably also correspond to recent crustal ramp anticlines this implies that the wide mountainous upper crustal wedge making the northeastern edge of the tibet qinghai plateau is detached from the underlying lower crust and upper mantle

late quaternary slip rate on the oak ridge fault transverse ranges california implications for seismic risk abstract the slip rate on the oak ridge fault at south mountain in the densely populated lower santa clara valley california is estimated as 6 15 to 12 3 mm yr since the end of saugus deposition at 0 4 to 0 2 ma slightly lower slip rate estimates result from a different projection of the top of the saugus in the hanging wall block of the fault an assumed displacement of 3 m per event gives an average recurrence interval of 250 to 500 yr with the major uncertainty being the age of the top of the saugus displacement of 2460 m of the top of the saugus at south mountain includes piercing point displacement and distributed displacement from drag folds near the fault both are the near surface expression of faulting at potential mainshock depths beneath well control slip rates for the nearby san cayetano and red mountain faults are not well constrained because post miocene strata are absent in their hanging wall blocks but available evidence suggests that their slip rates are about the same as that for the oak ridge fault earthquake return times measured in hundreds of years have been shown by others for the 1971 san fernando fault and a small fault near the red mountain fault and comparable return times are suggested for a normal fault in the hanging wall block of the oak ridge fault a trench on the harmon alluvial fan near the ventura county government center revealed evidence of cracks filled with sediments from below suggesting liquefaction during an earthquake the lower santa clara valley has not had a large damaging earthquake in 200 years of record keeping but one is expected there in the near future the december 21 1812 earthquake may have occurred on the offshore continuation of the oak ridge or red mountain fault

early cretaceous uplift and erosion of the northern appalachian basin new york based on apatite fission track analysis abstract the thermal history of outcropping devonian sediments of the northern appalachian basin new york has been investigated using fission track analysis of detrital apatites from 57 sandstone samples based on lengths and apparent age measurements using fission tracks in apatite it is concluded that lower devonian sediments presently at the surface in the catskill region were cooled rapidly from temperatures higher than about 110 c during early cretaceous times 120 140 ma ago in the western part of new york wellsville buffalo data from late devonian sediments are consistent with cooling at the same time as that identified for the catskill region but from lower temperatures in the range of approximately 80 110 c the maximum temperature these sediments experienced since deposition for a pre uplift paleogeothermal gradient of 25 35 c km the confined track length data indicates uplift and erosion of 2 3 km for western new york and greater than 3 4 km for the catskill region a differential uplift pattern which is consistent with the historical stratigraphic data from the region this conclusion is at variance with earlier interpretations put forth by others rapid broad scale uplift and erosion of the scale identified imply that large volumes of sediment could have been supplied from the northern appalachian basin during the early cretaceous this timing for the dominant post devonian cooling phase in the basin is not accounted for by recent models of the tectonic evolution of the appalachian orogen but is compatible with the change from carbonate to siliciclastic deposition in the atlantic coastal plain it is suggested that this style of broad regional uplift without significant deformation is characteristic of a tectonic regime associated with and subsequent to continental rifting apatite fission track analysis is shown to be a basic tool in providing fundamental limits for thermal history assessment in regional tectonic problems

active flexural slip faulting controls exerted by stratigraphy geometry and fold kinematics flexural slip plays an important role in accommodating fold growth and its topographic expression flexural slip fault fsf scarps may be one of the most commonly occurring secondary structures in areas dominated by active thrusts and folds where fsf scarps are present and what factors control their occurrence however are typically poorly known through an investigation of clearly expressed fsf scarps well preserved fluvial terraces and well exposed bedrock at eight sites in the pamir tian shan convergent zone and kuche fold belt nw china we summarize the most favorable conditions for active flexural slip faulting our study yields six key results first flexural slip operates commonly in well layered beds although uncommonly can occur in massive poorly layered beds as well second in well layered beds the slip surface is commonly located either a close to the contact of competent and incompetent beds or b within thin incompetent beds third fsf scarps are always found overlying steep beds with dips of 30 100 fourth slip surfaces are typically spaced between 10 and 440 m but can reach up to 600 m fifth fsf scarps at most sites can be observed far away from the hinge migrated fold scarps suggesting that compared to hinge migration limb rotation is generally required to accumulate flexural slip and produce associated topographic scarps finally a higher regional convergent rate seems to facilitate the creation of fsf scarps more often than lower rates whereas well preserved old terraces capped by thin deposits are more likely to record fsf scarps than unevenly preserved young terraces with thick sedimentary caps

hinge migrated fold scarp model based on an analysis of bed geometry a study from the mingyaole anticline southern foreland of chinese tian shan 2015 american geophysical union all rights reserved fold scarps a type of geomorphic scarp formed by folding mechanisms of hinge migration or limb rotation serve to delineate both fault bend characteristics and folding histories which can in turn illuminate tectonic processes and seismic hazards associated with thrust systems because the subsurface geometry of folds is commonly difficult to determine existing fold scarp models which rely on both the fold type and its causative fault geometries remain uncertain with respect to the kinematic evolution of a given fold in this paper we develop a model to illustrate that irrespective of specific fold type and subsurface geometries fold scarp growth in the mechanism of hinge migration can be successfully reconstructed based on analyses of bed geometry this model reveals that the underlying bed dips and the ratio of hinge migration distance hinge width control the fold scarp shape and slope during initial growth ratio 1 the slope reaches a maximum which solely depends on underlying bed dips the scarp height however is independent of the hinge width and can be used to quantify folding magnitude application of our model to fold scarps in the mingyaole anticline in the southern foreland of chinese tian shan indicates that the modeled fold scarp geometry can roughly match with field observations the mingyaole shortening rate is estimated to be 5 0 mm a since 15ka such that this single fold has accommodated about half of the regional convergence during the holocene key points folding through curved hinges creates a distinctive fold scarp geometry scarp slope relies on ratio of hinge migration width but scarp height is not shortening rate of the mingyaole fold is estimated to be 5 0 mm a since 15 ka such that this single fold has accommodated about half of the regional convergence during the holocene

active fault hazard in southern california ground rupture versus seismic shaking there are exceptions to the common assumption that surface rupture by faulting is always accompanied by a damaging earthquake near ventura california faults of the oak view ojai area and orcutt and timber canyons displace late pleistocene and or holocene alluvial materials and soils by several tens of metres the movement is almost exclusively along bedding planes in response to flexural slip during folding therefore these faults constitute a ground rupture hazard however because they do not extend downward to rocks of high shear strength which could store large amounts of elastic strain energy they do not constitute a seismic shaking hazard in contrast the oak ridge fault near the coast does not cut strata shallower than 1 250 to 1 500 m below the surface although pressure ridges attest to recent activity similarly the newport inglewood fault between long beach and inglewood oil fields in the los angeles basin is expressed at the surface as a line of anticlinal hills rather than a throughgoing fault historical faulting at inglewood and possibly at rosecrans and dominguez oil fields may be related to oil field development rather than earthquake related displacement those portions of the oak ridge and newport inglewood faults pose seismic shaking hazard but not ground rupture potential from earthquakes alone for planning purposes it is necessary to distinguish faults with only ground rupture potential or with only seismic shaking potential from the well known faults with potential for both

structural evolution of carpinteria basin western transverse ranges california the pleistocene carpinteria basin is an east trending northward verging faulted syncline containing up to 4 000 ft 1 220 m of partially intertonguing santa barbara and casitas formations deposited on previously folded pre pleistocene strata with up to 80 discordance structures subcropping against the unconformity indicate that most of the deformation in the santa ynez mountains prior to deposition of the pleistocene santa barbara formation was by folding during that time the carpinteria area was the northern margin of the offshore ventura basin subsequent uplift during middle to late pleistocene time isolated the carpinteria basin north of the red mountain fault quaternary faults in the area are either south dipping reverse faults related to bedding slip in pre pleistocene strata or north dipping reverse faults that truncate bedding and are seismogenic the rincon creek fault dips 35 to 60 south steepening to nearly vertical at depth where it apparently passes into bedding within the sespe formation the rincon creek fault offsets late pleistocene marine terraces and is itself deformed by the red mountain fault the red mountain fault dips 55 to 63 north at the surface and steepens to 75 north with depth it also steepens westward south of the summerland offshore oil field to 85 north vertical separation decreases westward from 14 750 ft 4 500 m north of the rincon field to 1 150 ft 350 m at rincon point and 330 ft 100 m south of summerland the main branch of the red mountain fault offsets a 45 000 year old marine terrace but not a 4 500 year old terrace the summerland offshore oil field is situated within a disharmonically folded anticline in which severely deformed structurally incompetent miocene mudstone overlies broadly folded competent oligocene sandstone because the anticline formed after deposition of the santa barbara formation oil could not have migrated into this field until middle to late pleistocene time

can flexural slip faults related to evaporite dissolution generate hazardous earthquakes the case of the grand hogback monocline of west central colorado a paleoseismological investigation of flexural slip faults related to interstratal evaporite dissolution suggests that such gravitational structures might have the potential to generate earthquakes with damaging magnitude the carbondale collapse center in the southern rocky mountains of colorado is a morpho structural depression of 1200 km 2 where miocene volcanic rocks are downdropped as much as 1200 m due to interstratal dissolution of halite bearing evaporites on the western margin of the collapse center the debuttressing effect related to active evaporite dissolution drives unfolding of the steeply dipping late laramide grand hogback monocline accompanied by displacement on bedding parallel faults these flexural slip faults rupture unconformable miocene basalts and quaternary mantled pediments generating conspicuous half graben depressions bounded by antislope fault scarps parallel to the underlying strata of the monocline two trenches dug across flexural slip fault scarps developed in each stratigraphic marker basalt cap mantled pediment revealed unexpected evidence of multiple late quaternary faulting events e g faulted colluvial wedge sharp unconformities with displacement per event values of 1 m three faulting events were inferred from the trench dug in the pediment w around 6

a barremian aptian early cretaceous north american paleomagnetic reference pole four intrusive complexes from southern maine containing rocks ranging in composition from olivine gabbro to granodiorite were sampled to determine an early cretaceous reference pole for the north american plate sampling included the lebanon diorite the tatnic complex alfred complex and the cape neddick complex dated radiometrically by the 40ar 39ar or k ar method on biotite between 125 and 120 ma the majority of samples have unblocking temperatures between 550 c and 580 c and curie temperatures between 550 c and 570 c indicating that the dominant remanence carrier is titanomagnetite which crystallized from the magma or was produced by deuteric oxidation alteration during post magmatic cooling titanomagnetite occurs as discrete c1 c3 grains as exsolution or oxidation exsolution lamellae in the silicates as a product of magmatic reactions forming magnetite ilmenite symplectites with pyroxene or olivine or as ultrafine dust hemo ilmenite is present as a minor component in the lebanon diorite and in this case the remanence direction carried by the hemo ilmenite is indistinguishable from the titanomagnetite detailed petrographic observations and rockmagnetic experiments demonstrate that despite the coarse average grain size of the oxides in these rocks which might lead one to expect unstable magnetic remanence the magnetic remanence in most samples is extremely stable and caused by single domain or pseudo single domain behavior in magnetite containing fine oxidation exsolution lamellae of ilmenite the tatnic and alfred intrusions are of normal polarity d 333 i 62 pole 70 5 n 210 3 e a95 3 7 n 24 sites the cape neddick and lebanon intrusions are of reversed polarity d 161 i 57 pole 73 7 n 185 7 e a95 5 0 n 17 sites together these intrusions yield a mean paleomagnetic pole at 72 2 n 198 9 e a95 3 3 n 41 sites the well characterized reversed polarity recorded in the precisely dated cape neddick complex may represent the best age for the magnetic polarity chron m0r the last reversal before the onset of the cretaceous normal superchron at the barremian aptian stage boundary

determination of domain structure in igneous rocks by alternating field and other methods abstract af alternating field demagnetization arm anhysteretic remanent magnetization and strong field hysteresis properties of a large collection of mostly continental igneous rocks are reported here the collection included rocks whose magnetic carriers were believed from previous work to be of one of three types md multidomain sd psd single domain pseudo single domain or a bimodal mixture of md grains e g discrete opaques and sd psd material e g silicate inclusions two series of subaerial basalts with a full range of deuteric oxidation classes included examples of all three classes of behaviour sd psd rocks have relatively hard inflected af decay curves decay rate initially increasing then decreasing md rocks have soft exponential like decay curves and bimodal rocks have a combination of these characteristics relative hardnesses of normalized decay curves of remanences acquired in weak intermediate and strong fields the lowrie fuller test are also distinctively different for the three classes and the results support the theory developed in an accompanying paper 1 that lowrie fuller characteristics are an expression of the shapes of decay curves the lowrie fuller test although its result can be expressed as a numerical parameter is not capable of fine scale classification of domain structure or grain size the shape of the arm induction curve does have a quasi continuous variation with grain size however the parameter ar j rs initial anhysteretic susceptibility normalized to saturation remanence which is easily measured with standard paleomagnetic instrumentation is potentially useful for magnetic granulometry although ar itself was not diagnostic of grain size

seismic images of a collision zone offshore nw sabah borneo multichannel reflection seismic data from the southern south china sea refraction and gravity modelling were used to investigate the compressional sedimentary structures of the collision prone continental margin off nw borneo an elongated imbricate deepwater fan the toe thrust zone bounds the northwest borneo trough to the southeast the faults separating the individual imbricates cut through post early miocene sediments and curve down to a carbonate platform at the top of the subsiding continental dangerous grounds platform that forms the major detachment surface the age of deformation migrates outward toward the front of the wedge we propose crustal shortening mechanisms as the main reason for the formation of the imbricate fan at the location of the in the past defined lower tertiary thrust sheet tectonostratigraphic province a high velocity body was found but with a much smaller extend than the previously defined structure the high velocity structure may be interpreted either as carbonates that limit the transfer of seismic energy into the sedimentary layers beneath or as paleogene crocker sediments dissected by remnants of a proto south china sea oceanic crust that were overthrust onto a southward migrating attenuated continental block of the dangerous grounds during plate convergence