listric faulting and continental rifting in western mexico a paleomagnetic and structural study abstract the western trans mexican volcanic belt is characterized by several large scale tectonic depressions which recently have been interpreted in terms of active rifting and independent motion of the jalisco block in this paper we report results for the northwestern part of the jalisco block within the ameca tectonic depression the trend of the central part of the depression changes where it is bounded by the ameca river fault there the fault which is characterized by a steep high scarp and a pronounced surficial curvature bounds the pleistocene amatlan de canas volcanic field which consists of small eroded volcanic structures and mafic lava flows intercalated with conglomeratic talus debris the sinuous pattern of outcrop of the fault indicates that the fault plane dips toward the volcanic field the conglomeratic units are abruptly truncated at the fault contact and generally dip 12 to 20 toward the fault plane paleomagnetic results for the young volcanic units give a mean direction which is interpreted to be the result of 12 to 20 of tilting toward the fault regional listric faulting block sliding of the entire block occupied by the volcanic field is postulated to explain the structural stratigraphic and paleomagnetic patterns observed listric faulting occurs within a transtensional tectonic environment related to the southwesterly motion of the jalisco block as it rifts away from mainland mexico

southward migration of continental volcanic activity in the sierra de las cruces mexico palaeomagnetic and radiometric evidence new palaeomagnetic data for 30 sites 271 samples and k ar data from five units in the sierra de las cruces western basin of mexico provide constraints on the spatial temporal evolution of arc magmatism in the central trans mexican volcanic belt the normal and reversed directions show a polarity pattern with a consistent spatial zonation perpendicular to the nnw sse trend of the range the magnetostratigraphy and k ar dates indicate that volcanic activity in the sierra de las cruces migrated southeastward at a mean rate of 1 6 cm a between 3 6 and 1 8 ma and that the rate of migration may have been higher up to 4 cm a during the gauss chron normal and reversed directions pass the reversal test at a 95 confidence level the mean plio quaternary palaeomagnetic direction for sierra de las cruces is d 350 7 i 30 6 n 25 k 30 7 95 5 3 the declination deviates to the west of the expected direction which suggests that small counterclockwise rotations could take place during formation of the sierra de las cruces volcanics

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

palaeomagnetism of the western and central sectors of the trans mexican volcanic belt implications for tectonic rotations and palaeosecular variation in the past 11 ma summary new palaeomagnetic data from the central and western sectors of the trans mexican volcanic belt tmvb are presented and analysed for palaeosecular variation psv and tectonic rotations fifty one sites with geochronological control were collected from selected volcanic lava flows which cover the temporal and spatial activity in the magmatic arc for the past 11 ma rock magnetic experiments reveal that generally ti poor titanomagnetites sometimes maghemitized are the magnetic carriers of the characteristic remanent magnetizations after analysis of detailed progressive demagnetization data 47 mean site directions are determined in addition data from previous regional palaeomagnetic study in the eastern tmvb sector 53 sites are re analysed palaeomagnetic data are grouped according to geographic distribution three structural sectors and according to age late miocene pliocene or quaternary to avoid discarding tectonic effects the less stringent criterion i e a fixed cut off angle to the mean equal to 45 was applied to identify transitional data affecting four sites palaeomagnetic results backed by positive reversal tests indicate no palaeomagnetically detectable vertical axis block rotations in the study areas the nearly geocentric axial dipole gad inclinations suggest no significant tilting effects and negligible quadrupolar contributions since the mean palaeomagnetic directions for the study areas do not differ from reference directions from the north american polar wander path two data sets for 11 5 and 5 0 ma are tested for geomagnetic purposes virtual geomagnetic poles vgps were selected using both fixed 45 and optimum variable cut off angles this resulted in vgp dispersions that increase back on time and are consistent with those obtained from globally distributed observations at the site latitude for their respective age ranges and cut off criteria palaeomagnetic data from late miocene and pliocene tmvb rocks can be considered in the databases for time averaged field taf and palaeosecular variation from lavas psvl analyses and geomagnetic field geometry characterization during the past 11 ma

geochemistry of evolved magmas and their relationship to subduction unrelated mafic volcanism at the volcanic front of the central mexican volcanic belt abstract this study reports new geochemical and radiogenic isotope data on hy normative pleistocene to recent 87 sr 86 sr 0 70370 0 70469 143 nd 144 nd 0 51278 0 51289 206 pb 204 pb 18 64 18 72 207 pb 204 pb 15 59 15 62 and 208 pb 204 pb 38 39 38 52 these isotopic ratios are generally similar to those for the associated mafic rocks although some differences do exist between them particularly for the 87 sr 86 sr ratios that are somewhat higher and 143 nd 144 nd that are slightly lower for the evolved rocks the available geochemical evidence does not support the generation of the scn evolved magmas by simple fractional crystallization fc or even by assimilation coupled with fractional crystallization afc of such mafic magmas nor by direct slab melting or indirect fluid transport to the mantle participation of the subducted cocos plate the viable mechanism seems to be that the most evolved andesitic and dacitic magmas in the scn were produced by partial melting of a heterogeneous mafic granulite source in the lower crust the magmas of intermediate compositions may reflect magma mixing processes between the most evolved andesitic and dacitic magmas generated in the lower crust and the mantle derived mafic magmas basalt and basaltic trachy andesite at the volcanic front of the central part of the mvb these results have direct bearing to other volcanic areas with complex tectonic setting

the relationship between quaternary volcanism in central mexico and the seismicity and structure of subducted ocean lithosphere late quaternary volcanism in central mexico is related to the subduction of young ocean lithosphere at the middle america trench along arc variations in seismicity volcano structure and composition of volcanic products bear a remarkable correlation with the age and structural framework of the downgoing slab morphological and petrographic characteristics of major volcanoes within the trans mexican volcanic belt tmvb serve to distinguish two calc alkaline subprovinces 1 a western arc averaging 60 km in width associated with aseismic subduction of the rivera plate the main cones of this region are dominated by two pyroxene andesites comprise volumes 70 km 3 and stand less than 3 000 m above sea level 2 a broad central and eastern arc related to subduction of a gently inclined segment of the cocos plate bounded by the rivera transform and the tehuantepec ridge major volcanic edifices possess summit elevations in the range 4 000 to 6 000 m have appropriately larger volumes typically 200 km 3 and are constructed with a high proportion of amphibole bearing lavas the boundary between these subprovinces is marked by a north south oriented structural depression the colima graben and it coincides with a 100 km offset in the volcanic front extensional tectonism in the colima graben accompanied by mixed calc alkaline and alkaline volcanism of potassic affinity is likely related to a hinge type transform fault which marks the cocos rivera plate juncture in the downgoing slab a third segment of ocean floor is presently interacting with continental lithosphere south of the gulf of tehuantepec where quaternary volcanism is weakly developed within a tectonically complex region that marks the diffuse cocos noam caribbean triple junction the northern limit of this triple junction is defined by the seismically active isthmus fault which may be related to alkaline volcanic activity at san andres tuxtla a tectonic reconstruction based on the evolution of oceanic crust reveals that the distribution of intermediate depth earthquakes along the arc is directly dependent upon the age of the subducted slab ocean lithosphere younger than approximately 20 m y is subducted aseismically at convergence rates approaching 9 cm yr the length of the inclined seismic zone indicates that the time constant for thermal relaxation in the slab is approximately 4 m y the tmvb overlies the aseismic extension of this young ocean lithosphere several aspects of this study have a bearing on the segmented nature of converging margins in general 1 the tectonic evolution of the ocean floor may determine the nature of segmentation at the site of subduction 2 the complete record of volcanism in the tmvb over the past million years can be related to the present plate configuration 3 alkaline and calc alkaline volcanism have developed contemporaneously at a converging plate margin 4 lineaments in volcanic arcs may reflect the structural complexity of the crust rather than segment boundaries in the subducted slab

palaeomagnetism of the puerto vallarta intrusive complex and the accretion of the guerrero terrain mexico abstract the batholith exposed around puerto vallarta is part of the guerrero tectonostratigraphic terrain presumably accreted to mexico during the laramide orogenic phase fourteen sites on this batholith were studied palaeomagnetically and geochemically nine sites define a paleopole at 76 6 n 181 8 e with 95 6 6 coinciding with the reference pole for stable north america it is proposed that no important movements of the region relative to north america have occurred since emplacement of the intrusive rocks 80 ma ago this would mean that the guerrero terrain was accreted to the mexican continental margin before that time geochemical data support this interpretation characterizing the rocks as volcanic arc granites intruded at an active continental margin

hypocentral trend surface analysis probing the geometry of benioff zones a hypocentral trend surface is a continuous function of latitude and longtitude fitted by least squares to a set of hypocenters so that it predicts depth to the middle of a benioff zone in this paper we take a relatively simple approach to hypocentral trend surface analysis the hypocentral trend surface is constructed from a spherical surface harmonic expansion whose coefficients are selected so as to minimize the standard vertical deviation between hypocenters and trend surface the vertical deviation of a hypocenter from the hypocentral trend surface is called its residual consideration of a hypocentral trend surface cannot be divorced from consideration of the associated residuals and their spatial distribution any analysis of benioff zone geometry trades off presumed thickness of the zone with its presumed shape this trade off can be investigated by examining suites of candidate hypocentral trend surfaces together with their associated residual distributions generated by varying the number of degrees of freedom available to the trend surface hypocentral trend surfaces are generated for three high quality sets of hypocenters obtained by local seismic networks in honshu japan cook inlet alaska and south peru hypocenters beneath honshu generate a bimodal distribution of residuals about their trend surface the vertical separation of the upper and lower sheets of this double benioff zone averages about 33 km no double benioff zone configuration is discovered below cook inlet or south peru regional trend surfaces are established for intermediate depth teleseismic data from south and middle america in south america 0 40 s teleseismic data suggest that along strike transitions between flat and moderately steeply dipping sections of the benioff zone are achieved by flexure of a coherent slab rather than by fragmentation of the slab into tear bound flaps or separate tongues of lithosphere hypocentral trend surface analysis of teleseismic data from south peru suggests a geometry similar to that indicated by hypocentral trend surface analysis of the local data of hasegawa and sacks an eastward shallowing benioff zone beneath southwest mexico is closely associated with the geometry of the trans mexican volcanic arc the benioff zone beneath central america is more steeply dipping the flat slab steep slab transition near 96 w may relate to the difference in age of oceanic lithosphere on either side of the tehuantepec ridge while the strong flexure of the subducted nazca plate beneath ecuador is closely associated with the carnegie ridge the nazca ridge extropolates slightly north of the flexure in subducted lithosphere beneath south peru the flexure appears instead more closely associated with the pronounced inflection in the map view shape of the leading edge of the overriding south american plate

active faults and state of stress in the central part of the trans mexican volcanic belt mexico 1 the venta de bravo fault the central part of the trans mexican volcanic belt is being deformed by seismically active west east striking normal faults that cause pronounced scarps structural and seismic along strike discontinuities allow the following segmentation 1 the western segment the area between morelia and the los azufres volcanic center is characterized by 25 km long fault scarps that define the cuitzeo graben and the santa ana maya horst which has a relief 850 m with respect to the cuitzeo graben the faults cut several quaternary scoria cones as well as alluvium 2 the central neotectonic segment extends between maravatio in the west and altamirano volcano in the east the most prominent structure of this segment is the 45 km long venta de bravo fault with a relief 300 m the february 22 1979 mb 5 3 maravatio earthquake occurred on this fault the late quaternary vertical slip rate along the easternmost part of the venta de bravo fault is estimated at 2 mm yr the lateral extension of the segment is defined by the limits of the venta de bravo fault and the eastern termination of the cuitzeo graben 3 the eastern segment is made up of the acambay graben the limit between the central and eastern segments is defined by an extensional jog between the venta de bravo and pastores faults which indicates a left lateral strike slip component the most recent rupture of this segment occurred in the november 19 1912 ms 6 9 acambay earthquake that caused vertical displacements 50 cm along the faults flanking the graben stress determinations based on quaternary scoria cone alignments the focal mechanism of the 1979 earthquake and the inversion of striation measurements indicate a stress field with sv sene snnw the fault striations as well as the focal mechanism indicate a minor but consistent left lateral strike slip component the observed stress and deformation can be explained by the superposition of two sources of stress isostatically compensated surface loads related to the high elevation of the trans mexican volcanic belt probably cause horizontal deviatoric tension and the observed crestal normal faults which strike parallel to the axis of the volcanic belt the observed left lateral strike slip component on the other hand can be explained by compressional far field stresses caused by loads at the boundary between the cocos and north american plates the orientation of the faults with respect to the direction of plate convergence is a prefered orientation for left lateral slip along the observed faults alternatively the observed left lateral strike slip component may accommodate trench parallel motion of crustal blocks in southwestern mexico

bed slope effects on turbulent wave boundary layers 2 comparison with skewness asymmetry and other effects 1 a numerical model solving incompressible reynolds averaged navier stokes equations combined with a two equation k model for turbulence closure is used to systematically compare the relative strength of bed shear stress quantities and boundary layer streaming under wave motions from four contributions believed to play a prominent role in cross shore boundary layer and sediment transport processes 1 converging diverging effects from bed slope 2 wave skewness 3 wave asymmetry and 4 waves combined with superposed negative currents intended to loosely represent for example return currents or undertow the effects from each of the four components are isolated and quantified using a standard set of bed shear stress quantities allowing their easy comparison for conditions representing large shallow water waves on steep slopes the results suggest that converging diverging effects from beach slope may make a significant onshore bed load contribution generally however the results suggest wave skewness in addition to conventional steady streaming as the most important onshore contribution outside the surf zone streaming induced within the wave boundary layer is also investigated for each component and skewness and asymmetry are demonstrated to promote largely negative streaming velocities consistent with earlier work for hydraulically smooth cases however a thin region of positive streaming is revealed in the viscous sublayer which is effectively absent in hydraulically rough conditions cross shore boundary layer dynamics are generally acknowledged to be a complicated combination of numerous competing physical processes and this work is among few to systematically compare a number of these it is hoped that the results will be useful in assessing which of the contributions considered will be most prominent for a given set of local wave and beach slope conditions

geology of los azufres caldera mexico and its relationships with regional tectonics ferrari l gardufio v h pasquar6 g and tibaldi a 1991 geology of los azufres caldera mexico and its relationships with regional tectonics in s p verma editor calderas genesis structure and unrest j volcanol geotherm res 47 129 148 the los azufres geothermal field is one of several silicic centres of the mexican volcanic belt mvb for which a caldera structure was suggested geological and structural surveys in a wide area surrounding los azufres reveal that this complex is situated in an area of unusual concentration of acid volcanism consisting of four pyroclastic units and several dome complexes although a complete caldera structure is not morphologically evident several facts suggest a long collapse history at los a zufres the geothermal field lies at the centre of a subcircular depressed area 27 26 km in size filled by a distinctive fluviolaeustrine sequence middle to late miocene rocks bound to the south and to the north of this depression and are encountered only at depth inside it four large ignimbritic suites of latest miocene and pliocenc age outcrop outside the depression volume estimations of these pyroclastic products are comparable with the missing volume of the depressed area no alternative caldera structures exist in a radius of 200 km from los azufres dacitic to rhyolitic lavas principally extruded as dome complexes were emplaced inside this depression during the pleistocene los azufres is therefore interpreted as a nested caldera of latest miocene and pliocene age the recent regional tectonic evolution of the central sector of mvb comprises a late miocene early pliocene left lateral transcurrent phase followed by a late pliocene quatern ary transtensional one the silicic volcanism occurring between 6 1 and 2 8 ma can be linked to the first phase while in the adjacent areas of mvb a volcanic hiatus can be recognized the following transtensional phase reached the los azufres area only during the pleistocene and disrupted the caldera structure normal faults developed during this period controlled the uprising of basic magma which partly interacted with the remaining differentiated one and produced part of the recent intracaldera cycle

experimental observations of tsunami induced scour at onshore structures tsunami inundation of the coastal environment can induce scour at structure foundations leading to failure a series of experiments are made using a unique pneumatic long wave generator to generate tsunami wave periods of 25 147 s equating to 3 17 3 min at 1 50 froude scale the waves propagate over a sloping bathymetry and impinge upon a square structure founded onshore in a flat sediment bed flow velocity height and scour depth are recorded as a function of time during tsunami inundation the rate of scour is observed to be time dependent equilibrium which is not attained is argued to be an inappropriate measure for time dependent transient flows such as tsunami in which the flow velocity depth and direction are variable the maximum scour depth is recorded and critically is observed not to be equal to the final depth due to significant sediment slumping when flow velocities reduce in the latter stages of inundation current and wave scour predictor equations over predict the scour while the asce 7 16 method under predicts comparisons with available data in the literature show longer inundation durations increase the amount of scour

geology of the northern colima and zacoalco grabens southwest mexico late cenozoic rifting in the mexican volcanic belt the south trending northern colima graben intersects two other grabens the northwest trending zacoalco graben and the east trending chapala graben to form a rift rift rift triple junction 50 km south southwest of guadalajara in southwest mexico the northern colima graben represents the northernmost extension of the 190 km long 20 to 65 km wide colima rift which overlies the projection of the subducting boundary of the rivera and cocos plates and represents a rift of probable pliocene pleistocene age of the east trending mexican volcanic belt mvb associated with the northernmost 90 km of the rift there are 4 6 m y old to holocene alkaline volcanic rocks rarely found in continental volcanic arc environments and which are more typically associated with either continental intraplate rifting or oceanic islands these alkaline lavas have erupted side by side with 10 m y old to holocene calc alkaline lavas that represent typical arc volcanism it is proposed that the grabens are associated with the initiation of a rifting of a portion of southwest mexico away from the mexican mainland similar to the rifting of baja california away from the mexican mainland the calc alkaline k ar dates are consistent with post mid miocene development of east trending mvb volcanism

tectonics along the trans mexican volcanic belt according to palaeomagnetic data abstract subduction of the cocos plate beneath the southern mexico continental margin takes place at the middle america trench volcanism of calc alkaline character occurs along the e w trending trans mexican volcanic belt tmvb which includes several active stratovolcanoes monogenetic cinder cone fields and silicic centres however a direct association between the volcanism and plate subduction has been questioned for a number of reasons including the oblique orientation of the tmvb with respect to the trench most palaeomagnetic directions for the cretaceous to quaternary within or immediately to the south of the tmvb appear discordant the declinations are rotated anticlockwise relative to expected declinations this angular divergence may be explained by rotations of crustal blocks by different degrees in response to large scale left lateral shear across central mexico the fault system may have acted as an efficient structural control for the rise of magmas which may explain some of the peculiarities of the tmvb

northern east pacific rise evolution from 25 m y b p to the present the northeast pacific topography and magnetic lineations 25 m y b p to the present record the traces of three major spreading reorganizations only one spreading center is observed today but there is evidence for several ephemeral episodes of twin spreading accompanying the evolution from an extensive pacific guadalupe plate boundary to a much shortened pacific cocos and pacific rivera plate boundary the 25 m y b p plate reorganization culminated with the formation of the guadalupe plate bound by the murray fracture zone to the north and the cocos nazca spreading ridge to the south between 25 and 12 5 m y b p spreading continued while the plate retained the same general outline the 12 5 11 m y b p reorganization resulted in the creation of a much shortened pacific cocos plate boundary located in its early stages over the mathematician seamounts and a much reduced cocos plate the last reorganizaton 6 5 3 5 m y b p resulted in the abandonment of the mathematician spreading ridge as a pacific cocos plate boundary in favor of the east pacific rise

palaeomagnetic data from central mexico and their use for palaeosecular variation studies abstract three brunhes age palaeomagnetic data sets from central mexico exhibit different dispersion of virtual geomagnetic poles volcanics from the eastern part of the trans mexican volcanic belt result in a s f 14 8 this value is interpreted to represent an upper limit as these rocks probably are affected by localized tectonic rotations for a stalagmite s f 9 5 which is interpreted as a lower limit as the stalagmite is only 1200 years old and the data are strongly biased in time seventy four volcanic units from the chichinautzin formation result in s f 11 7 with confidence limits of 10 4 and 13 3 this dispersion is much lower than expected from palaeosecular variation models but corresponds to data obtained for other areas in or beneath the pacific basin e g the hawaiian islands

geochemical evidence for a lithospheric source for magmas from los humeros caldera puebla mexico abstract this study reports new geochemical and sr nd pb radiogenic isotope data on hy normative quaternary basaltic to rhyolitic volcanic rocks as well as two samples of mesozoic to palaeozoic intrusive igneous rocks from los humeros caldera lhc located in the eastern part of the mexican volcanic belt mvb the isotopic ratios of mafic basaltic rocks range as follows 87 sr 86 sr 0 703350 0 703387 143 nd 144 nd 0 512867 0 512888 206 pb 204 pb 18 849 18 866 207 pb 204 pb 15 593 15 598 and 208 pb 204 pb 38 490 38 491 these values are similar to the rift related mafic rocks from the volcanic front of the central mvb for comparison the entire suite from basalt b to rhyolite r in the lhc shows the following ranges 87 sr 86 sr 0 703350 0 704678 143 nd 144 nd 0 512702 0 512888 206 pb 204 pb 18 710 18 866 207 pb 204 pb 15 593 15 631 and 208 pb 204 pb 38 444 38 594 the basaltic rocks from los humeros do not show the characteristic negative nb anomaly with respect to ba and ce as is typical of such magmas in volcanic arcs the available geochemical and isotopic evidence does not support the generation of the lhc mafic magmas by direct slab melting nor by indirect fluid transport to the mantle participation of the subducted cocos plate instead they could be generated in the upper mantle from a lithospheric source the evolved andesitic to rhyolitic magmas could have been originated from such mafic magmas through assimilation coupled with extensive fractional crystallisation two different compositions representing lower lc and upper crusts uc as well as a hypothetical crust ha were tested as plausible assimilants for the assimilation coupled with fractional crystallisation process

el estribo volcanic complex evolution from a shield volcano to a cinder cone patzcuaro lake michoacan mexico abstract el estribo volcanic complex evc is located in the northern part of the michoacan guanajuato volcanic field within the trans mexican volcanic belt tmvb el estribo is located at the southern edge of the e w patzcuaro fault that belongs to the patzcuaro jaracuaro graben a western extension of the e w morelia acambay fault system stratigraphy geochronology chemistry and mineral assemblages suggest that the volcanic complex was constructed in two periods separated by a 100 ka volcanic hiatus a emission of lava flows that constructed a shield volcano between 126 ka and b mixed phreatomagmatic to strombolian activity that formed a cinder cone 28 ka the magmas that fed these monogenetic volcanoes were able to use the same feeding system the cinder cone itself was constructed by strombolian fallouts and remobilized scoria beds followed by an erosion period and by a mixed phreatomagmatic to magmatic phase strombolian fallouts ending with lava flows soft sedimentary deformation of beds and impact sags cross bedding as well as pitting and hydrothermal cracks found in particles support the phreatomagmatic phase the erupted magmas through time ejected basaltic andesitic lava flows 56 21 58 88 sio 2 that built the shield volcano and then basaltic andesitic scoria 57 65 59 05 sio 2 that constructed the cinder cone although they used the same feeding system the geochemical data and the mineral chemistry of the magmas indicate that the shield volcano and the cinder cone were fed by different magma batches erupted thousands of years apart therefore the location of el estribo volcanic complex along an e w fault that has generated two sector collapses of the shield volcano to the north may be directly linked to this complex redistribution of the magmatic paths to the surface our findings show that magmatic feeding systems within monogenetic volcanic fields could be long lived questioning the classic view of the monogenetic nature of their volcanoes and yielding information about the potential volcanic risk of these settings usually considered risk free

tectonic interpretation of the trans mexican volcanic belt discussion discussion sur l article de shurbet d h et cebull s e paru dans tectonophysics nld 1984 101 159 165

paleomagnetism of the miocene jantetelco granodiorites and tepexco volcanic group and inferences for crustal block rotations in central mexico reevaluation abstract miocene igneous rocks from a location in central mexico provide paleomagnetic data that allow re evaluation of previous data by urrutia fucugauchi 1981 that were interpreted to suggest a counter clockwise rotation of a crustal block by about 50 we sampled 31 sites from the miocene trondhjemitic chalcatzingo domes the tepexco volcanic group and the xalostoc diorite covering the same area of approximately 50 km 2 about 90 km se of mexico city magnetic analysis shows that these rocks contain magnetic phases of variable composition with curie temperatures characteristic for magnetite but often accompanied by lower curie temperature components magnetic hysteresis measurements point to the presence of psd particles and in combination these properties suggest that these rocks are also suitable for paleomagnetic study scanning electron microscope analysis supports the rock magnetic results indicating the presence of magnetite and high intermediate titanium titanomagnetite in many samples demagnetization experiments showed in most cases characteristic remanence directions of reasonable to good quality and for 26 sites mean directions could be determined of these 14 12 are of normal reverse polarity overall mean directions of normal and reverse sites are antipodal and pass a reversal test at the 95 probability level the paleodirection from 26 sites is d 348 1 e i 35 7 95 7 4 and the paleopole is located at lat 78 3 n long 184 2 e a 95 7 0 which is indistinguishable from the 20 ma reference pole for the stable north america plate these data do not support any tectonic deformation of the sampling area since the miocene based on the number of sites studied their rock magnetic characteristics and the quality of the magnetic remanence that results in a positive reversal test we consider our data to be reliable and we therefore suggest that this result supersedes that of urrutia fucugauchi 1981 the reason for this previous and much different result remains unknown

paleomagnetism of the acambay graben central trans mexican volcanic belt abstract paleomagnetic results for miocene to quaternary volcanic units of the acambay graben are used to investigate the neogene tectonic activity within the central sector of the trans mexican volcanic belt tmvb characteristic magnetization directions were obtained for 22 sites with an overall mean direction dec 176 7 inc 34 3 k 28 95 6 that is concordant with the expected direction calculated from north american reference data examination of site mean directions shows an apparent geographic pattern related to the intersection of the queretaro taxco fault zone with the e w border fault system of the graben this pattern is characterized by easterly and westerly declinations arranged into two broad groups the mean direction for group a easterly is n 9 dec 189 9 inc 28 5 k 53 and 95 4 8 the mean direction for group b westerly is n 13 dec 167 6 inc 36 2 k 38 and 95 4 3 group a sites cover two areas one inside the graben and the other immediately to the south including the pastores fault group b sites are distributed over broad nw and se areas across the graben which include the amealco caldera and epitacio huerta and acambay tixmadeje faults and also the venta de bravo fault eighteen units show reverse polarities and four units show normal polarity absence of normal polarity quaternary units suggests a pre brunhes age for units sampled k ar dating in the amealco caldera gives a range between 5 7 and 2 2 ma a comparison of group a and b mean directions with reference directions calculated from the north american 10 ma pole and the geographic pole gives rotation parameters between 9 1 5 3 and 9 9 4 6 and between 12 4 5 4 and 12 4 4 3 respectively these may be interpreted in terms of vertical axis rotations associated with regional left lateral shear in a normal strike slip fault environment which has characterized the acambay graben during the neogene

geomorphic assessment of active tectonics in the acambay graben mexican volcanic belt spatial variations of quaternary deformation and tectonic activity of faults along the acambay graben are assessed using geomorphic and morphometric approaches the acambay graben is an east west trending structure of apparent quaternary age located in the central part of the mexican volcanic belt which gives rise to pronounced scarps over a distance of about 80 km continuing tectonic activity in the acambay graben is confirmed by recent well documented seismic episodes the intensity of active tectonics has been interpreted through a detailed geomorphic study of the fault generated mountain fronts and fluvial systems the combined geomorphic and morphometric data provide evidence for relative variations in tectonic activity among the acambay graben faults geomorphic indices suggest a relatively high degree of tectonic activity along the venta de bravo and the acambay tixmadeje faults followed in order of decreasing activity by the pastores temascalcingo and tepuxtepec faults spatial variations within faults have also been identified suggesting a higher level of tectonic activity at the tips of the faults this pattern of variation in the relative degree of tectonic activity is consistent with field evidence and seismic data for the acambay graben geomorphic evaluation of the acambay graben faults suggests that the acambay tixmadeje and venta de bravo faults and specifically the tips of these faults and a central segment near the town of venta de bravo should be considered as areas of potentially high earthquake risk 1998 john wiley sons ltd

bed slope effects on turbulent wave boundary layers 1 model validation and quantification of rough turbulent results 1 a numerical model solving incompressible reynolds averaged navier stokes equations combined with a two equation k turbulence closure is used to study converging diverging effects from a sloping bed on turbulent oscillatory wave boundary layers bed shear stresses from the numerical model are first validated against hydraulically smooth and rough data from flat bed experiments in the form of wave friction factor diagrams the results show that the model provides acceptable accuracy for wave orbital amplitude to roughness length ratios a kn 20 validation against experimental measurements for steady streaming induced by a skewed free stream velocity signal is also provided we then simulate a series of experiments involving oscillatory flow in a convergent divergent smooth tunnel and a good match with respect to bed shear stresses and streaming velocities is achieved the streaming is conceptually explained using analogies from steady converging and diffuser flows a parametric study is undertaken to assess both the peak and time averaged bed shear stresses in converging and diverging half periods under rough turbulent conditions the results are presented as friction factor diagrams a local similarity condition is derived for relating oscillatory flow in a convergent divergent tunnel as considered herein to shoaling shallow water waves by matching spatial gradients in the free stream orbital length

active rifting in southwestern mexico manifestations of an incipient eastward spreading ridge jump a major ongoing episode of continental rifting has formed a system of three intersecting grabens in the western mexican volcanic belt these structures may mark the initial stage of an eastward spreading ridge jump beneath continental crust eastward ridge segment jumps have been propagating northward along this part of the east pacific rise for the past 12 m y but this is the first plate boundary reorganization event to involve continental crust since the separation of baja california in the miocene since pliocene time small volumes of unusual alkaline magmas have erupted in the graben areas in close association with the more abundant subduction related calc alkaline magmas erupted from andesitic stratovolcanoes the alkaline magmas are varieties found in zones of active rifting elsewhere in the world

steady streaming and sediment transport at the bottom of sea waves the flow and sediment transport in the boundary layer at the sea bottom due to the passage of surface waves are determined by considering small values of the wave steepness and of the ratio between the thickness of the boundary layer and the local water depth both the velocity field and the sediment transport rate are determined up to the second order of approximation thus evaluating both the steady streaming and the net wave averaged flux of sediment induced by nonlinear effects the flow regime is assumed to be turbulent and a two equation turbulence model is used to close the problem the bed load is evaluated by means of an empirical relationship as function of the bed shear stress the suspended load is determined by computing the sediment flux once the sediment concentration is determined by solving an appropriate advection diffusion equation the decay of the wave amplitude which is due to the energy dissipation taking place in the boundary layer is taken into account the steady streaming and the sediment transport rate at the bottom of sea waves turn out to be different from those which are observed in a wave tunnel u tube because of the dependence on the streamwise coordinate of the former flow in particular in the range of the parameters presently investigated the sediment transport rate at the bottom of sea waves is found to be always onshore directed while in a water tunnel u tube the sediment transport rate can be onshore or offshore directed

northern east pacific rise magnetic anomaly and bathymetric framework the oceanic crust in the eastern pacific between 7on and 30on and east of 127ow contains a fairly complete history of the spreading centers associated with the east pacific rise since 25 m y b p late oligocene in this paper we have summarized the seafloor spreading magnetic anomaly data and the bathymetric data that reflect the record of this tectonic history the well defined magnetic lineations north of the clarion fracture zone in the mouth of the gulf of california and on the east flank of the east pacific rise epr are carefully examined and used to provide a guide for interpreting the spreading pattern between the clarion and clipperton fracture zones southward of the rivera fracture zone over the mathematician ridge and over the entire epr east of the mathematician ridge between the rivera and siqueiros fracture zones the bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas the fracture zone bathymetry and the seafloor spreading magnetic lineations on the epr south of the rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations all these data provide a good record of the plate reorganizations in the middle miocene at magnetic anomaly 5a time 12 5 to 11 m y b p in the late miocene at magnetic anomaly 3 4 time 6 5 m y b p and in the pliocene at magnetic anomaly 2 3 time 3 5 m y b p several abandoned spreading centers including the mathematician ridge were left behind as a result of these reorganizations the mathematician ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5a

palaeomagnetism of the miocene jantetelco granodiorites and tepexco volcanic group and inferences for crustal block rotations in central mexico abstract palaeomagnetic directions isolated from seven sites in three bodies of the miocene jantetelco granodiorites and from two sites in two lava flows of the miocene tepexco volcanic group of central mexico point about 50 west of the expected miocene and middle tertiary geomagnetic field directions for the area both alternating field and thermal demagnetization experiments indicate that these remanent magnetizations are stable and free from any significant secondary overprinting the granodiorites present both normal and reverse polarity and at one site there is evidence of the coexistence of both polarities suggesting that the cooling of the intrusive spanned at least one polarity transition of the geomagnetic field the lavas present reverse polarity the angular deflection is attributed to the tectonic rotation of a crustal block about a pivot located somewhere nearby the tectonic rotation is likely related to lateral shear acting along the trace of the mexican volcanic belt the mechanism may have involved rotations of rigid crustal fragments while the area was translated in response to lateral shear along large regional bounding faults alternatively the rotations may have been associated with the formation of pullapart basins again in conjuction with the regional lateral shear

an integrated paleomagnetic study of rio grande de santiago volcanic succession trans mexican volcanic belt revisited abstract we carried out an integrated paleomagnetic rock magnetic and paleointensity study of miocene volcanic succession from the trans mexican volcanic belt tmvb north of guadalajara a total of 37 consecutive basaltic lava flows 326 oriented standard paleomagnetic cores were collected at lazo locality continuous susceptibility measurements with temperature and hysteresis experiments yield in most cases reasonably reversible curves with curie points close to that of pseudo single domain magnetite two geomagnetic reversals were observed in the 300 m thick composite section paleosecular variation was lower than the one observed in general during miocene it appears that the volcanic units have been emplaced during a relatively short time span of about 1 ma the mean paleomagnetic directions obtained from this study do not differ significantly from that expected for the middle miocene the mean paleomagnetic direction calculated from all data is i 31 1 d 354 6 k 124 and 95 2 1 n 37 seventy two samples with apparently preserved primary magnetic mineralogy and without secondary magnetization mostly belonging to reverse polarity chron were pre selected for thellier paleointensity determination the flow mean paleointensity values are ranging from 22 4 3 4 to 53 8 6 0 t and the corresponding virtual dipole moments vdms are ranging from 5 4 0 8 to 12 0 1 4 1022 a m2 this corresponds to mean value of 7 7 2 2 1022 a m2 which is close to present day geomagnetic field strength altogether our data suggest the existence of relatively high geomagnetic field strength undergoing low fluctuations

crustal velocity field of mexico from continuous gps measurements 1993 to june 2001 implications for the neotectonics of mexico 1 we combine velocities for 14 continuous gps stations spanning mexico and 173 additional continuous gps sites on the north american and pacific plates to study the large scale deformation of mexico the new station velocities which are derived from more than 6000 days of previously unused gps data provide the first ever view of the crustal velocity field of mexico key results are as follows 1 areas north of the mexican volcanic belt not including baja california move with the north american plate interior within the 1 2 mm yr 1 station velocity uncertainties station velocities for the mexican basin and range are consistent with no present day extension and yield an upper 95 limit of 1 3 mm yr 1 for any regional extension 2 south of the mexican volcanic belt five of the six sites move significantly relative to the north american plate all sites in the yucatan peninsula move toward the east at 3 4 mm yr 1 possibly defining an independent yucatan block 3 site velocities are consistent with limits of 0 4 mm yr 1 for present slip across the mexican volcanic belt 4 tampico on the gulf coast exhibits eastward motion consistent with gravity sliding known to occur in the adjacent mexican ridges fold belt 5 southeastward motion of la paz relative to the pacific plate is consistent with the hypothesis that the baja peninsula is not fully attached to the pacific plate 6 residual velocities for 160 north american plate gps stations outside of mexico exhibit no coherent regional patterns indicative of internal plate deformation

rans based simulation of wave induced sheet flow transport of graded sediments abstract a one dimensional vertical 1dv turbulence closure flow model coupled with sediment transport capabilities is extended to incorporate graded sediment mixtures the hydrodynamic model solves the horizontal component of the incompressible reynolds averaged navier stokes rans equations coupled with k turbulence closure the sediment transport description includes both bed and suspended load descriptions so called high concentration effects turbulence damping and hindered settling velocities are likewise included the sediment transport model treats the bed and suspended load individually for each grain fraction including effects associated with increased exposure of larger particles within a mixture the suspended sediment transport model also makes use of modified reference concentration approach wherein reference concentrations computed individually for each fraction are translated to a common level conveniently enabling use of a single computational grid for the simulation of suspended sediments parametric study shows that these two effects combine to help alleviate an otherwise systematic tendency towards over under predicted transport rates for fine coarse sand fractions the sediment transport model is validated against sheet flow experimental oscillatory tunnel measurements beneath velocity skewed wave signals and demonstrates similar accuracy transport rates generally within a factor of two for both graded and uniform sands the model is likewise validated against an extensive data set involving sheet flow transport beneath acceleration skewed wave signals limited to uniform sands it is then utilized to study potential effects of gradation on the net transport beneath such flows the simulations suggest that gradation effects can both increase as well as decrease the total transport rate depending largely on the behavior of the fine sand fraction the model is implemented within the matlab environment and is freely available upon request to the corresponding author

morphodynamics of prograding beaches a synthesis of seasonal to century scale observations of the columbia river littoral cell abstract findings from nearly two decades of research focused on the columbia river littoral cell crlc a set of rapidly prograding coastal barriers and strand plains in the u s pacific northwest are synthesized to investigate the morphodynamics associated with prograding beaches due to a large sediment supply from the columbia river the crlc is the only extensive stretch of shoreline on the u s west coast to have advanced significantly seaward during the late holocene since the last cascadia subduction zone csz earthquake in 1700 with associated co seismic subsidence and tsunami much of the crlc has prograded hundreds of meters however the rates of progradation and the processes most responsible for sediment accumulation vary depending on time scale and the morphological unit in question remarkably the 20th and early 21st century shoreline change rates were more than double the late prehistoric rates that include recovery from the last major csz event most likely due to an increase in sediment supply resulting from inlet jetty construction in some locations detailed beach morphology monitoring reveals that at interannual to decadal scale the upper shoreface aggraded about 2 cm yr subtidal sandbars migrated offshore and decayed while intertidal bars migrated onshore and welded to the shoreline the shoreline prograded about 4 m yr and 1 to 2 new foredune ridges were generated a detailed meso scale sediment budget analysis in one location within the littoral cell shows that approximately 100 m3 m yr accumulated between 12 m seaward limit of data and 9 m crest of landward most foredune gradients in alongshore sediment transport net onshore directed cross shore sediment transport within the surf zone and cross shore feeding from a shoreface out of equilibrium with forcing conditions are each partially responsible for the significant rates of sediment supplied to the beaches and dunes of the crlc during the observational period direct observations of beach progradation at seasonal to decadal scale are put in context of measured or inferred changes over time scales of decades to centuries

palaeomagnetism of late miocene to quaternary volcanics from the eastern segment of the trans mexican volcanic belt a systematic palaeomagnetic study in the eastern part of the trans mexican volcanic belt includes 39 miocene pliocene and quaternary volcanic rocks in the southeastern mexico basin sierra nevada and sierra de ri o fri o the altiplano area and the palma sola massif a total of 430 samples have been selectively demagnetized using mostly alternating field demagnetizing methods supplemented by thermal analyses most characteristic remanences are carried by low ti titanomagnetites with occasional titanohematites or slightly maghemitized low ti titanomagnetites of similar direction seven sites were discarded because they presented intermediate directions hydrothermal alteration or were remagnetized by lightning strikes the mean directions of 32 sites together with 24 sites from sierra de las cruces in the western mexico basin indicate rocks older than 2 ma are rotated some 10 counterclockwise with respect to quaternary rocks whereas there is no rotational difference between miocene and pliocene rocks statistical analyses between different regrouped populations confirm that the rotational pattern is due to the age of the volcanics rocks but not to their spatial distribution the quaternary mean direction from the three mexico basin ranges is consistent with the geographical reference pole in contrast the pliocene mean direction from volcanic rocks of the altiplano area and the sierra de las cruces is slightly rotated some 10 westwards with respect to the reference direction from north america no significant rotations have been observed in the eastern tmvb from the western mexico basin to the border of the altiplano between late miocene and late pliocene times it suggests that a very small counterclockwise vertical axis rotation may have been taken place in this segment of the tmvb between late pliocene and quaternary times comparisons of these results with a summary of the available palaeomagnetic data in the area indicate that the previously reported quaternary rotations are of questionable reliability and that the large counterclockwise rotations reported in cretaceous to miocene rocks probably took place before the late miocene these new palaeomagnetic data support the idea that the eastern tmvb since the late miocene has been a zone of extension with a little left lateral shear component

full scale cfd simulation of tsunamis part 2 boundary layers and bed shear stresses abstract this paper presents results from numerical simulations of the propagation and run up of full scale tsunamis using a reynolds averaged navier stokes model with emphasis on the resulting boundary layers and bed shear stresses spatial distributions of the shields and rouse parameters during run up and draw down show that for the tsunamis considered with a sediment grain size corresponding to medium sand considerable sediment transport shields parameter o 10 o 100 can be expected during run up and that the sediment transport can be expected to be dominated by suspended load the results likewise show that the expected sediment transport during draw down will similarly be considerable and dominated by suspension the tsunami induced boundary layers are monitored in time and the observed boundary layer thickness ranges from spanning only a small fraction of the water depth to spanning the entire depth the velocity profiles beneath the tsunamis are shown to have good correspondence with a logarithmic profile within the boundary layer similarly the bed shear stresses beneath the tsunamis are investigated and a new and simple engineering model is developed for predicting the temporal variation of the bed shear stress based only on a free stream velocity signal the new engineering model is shown give better predictions for the bed shear stress than a standard manning approach and is likewise shown in the appendix to produce reasonable predictions for shorter wind scale waves it is finally also shown how the temporal evolution of the boundary layer thickness can be predicted based on the free stream velocity signal alone the results presented here are part 2 of a larger study where part 1 presents the model validation and detailed descriptions of the run up process

tectonic interpretation of the trans mexicano volcanic belt abstract although some workers interpret subduction along the middle america trench to be directly associated with volcanism in the trans mexico volcanic belt geographic distribution of intermediate depth earthquakes analysis of sn propagation and a limited number of focal mechanism solutions suggest that the belt is largely independent of subduction these data in conjunction with the position of the volcanic belt with respect to known plate boundaries in the region suggest that the belt represents the incompletely developed northern boundary of a developing micro plate it is a zone of transtension that probably results from slightly different rates of absolute plate velocities in the region

modelling of sand transport under wave generated sheet flows with a rans diffusion model a 1dv rans diffusion model is used to study sand transport processes in oscillatory flat bed sheet flow conditions the central aim is the verification of the model with laboratory data and to identify processes controlling the magnitude and direction onshore offshore of the net time averaged sand transport the model is verified with a large series of measured net sand transport rates as collected in different wave tunnels for a range of wave current conditions and grain sizes although not all sheet flow details are represented in the 1dv model it is shown that the model is able to give a correct representation of the observed trends in the data with respect to the influence of the velocity wave period and grain diameter also detailed mean sediment flux profiles in the sheet flow layer are well reproduced by the model including the direction change from onshore to offshore due to a difference in grain size from 0 34 mm medium sand to 0 13 mm fine sand a model sensitivity study with a selected series of net transport data shows that the stirring height of the suspended sediment es ws strongly controls the magnitude and direction of the net sediment transport inclusion of both hindered settling and density stratification appears to be necessary to correctly represent the sand fluxes for waves alone and for waves a superimposed current the best agreement with a large dataset of net transport measurements is obtained with the 1dv rans model in its original settings using a prandtl schmidt number 0 5

two phase model for sand transport in sheet flow regime 1 we introduce a two phase model for sand transport in sheet flow regime this model uses a collisional theory and a k fluid turbulence closure to respectively model the sediment and fluid phase stresses the sediment stress closure adopts a balance equation of sediment particle fluctuation energy based on kinetic theory that incorporates two way interactions between fluid and sediment phases the fluid turbulence closure also considers the two way interaction between fluid turbulence and sand particles model data comparisons for the sheet layer for oscillatory flows in a u tube and for open channel flows demonstrate the model s predictive skill for steady open channel flows the fluid phase velocity follows closely the law of wall i e the log profile in which the von karman constant is reduced and the equivalent roughness is increased compared to the clear fluid flow conditions the model also provides information in the near bed region where the transition from the solid like to the fluid like behavior of sediment particles is resolved and both the bed load layer thickness and bed load transport rate can be evaluated for unsteady flows this model can predict time evolutions for sediment transport throughout the water column

paleomagnetism and tectonics from the late pliocene to late pleistocene in the xalapa monogenetic volcanic field veracruz mexico

statistical examination of the existence and relative motion of the jalisco and southern mexico blocks the continental lithosphere of mexico south of the trans mexican volcanic belt may consist of two lithospheric blocks the jalisco and southern mexico blocks the existence of these two blocks can be examined employing the statistical f test formulated to test for the presence of significant misclosure around a plate circuit to plate motion data derived from marine magnetic anomaly lineations earthquake slip vectors and transform fault azimuths the result of the f test applied to the pacific cocos north american plate circuit indicates that there is no significant at the 99 confidence level misclosure around this plate circuit therefore if the southern mexico block exists its motion relative to the surrounding lithospheric plates is too small to be resolved from these data in contrast the result of the f test applied to the rivera pacific north american plate circuit indicates the presence of a significant misclosure about the rivera pacific north american plate circuit the cause of which is uncertain one interpretation of the misclosure around the rivera pacific north american plate circuit is that it is due to the presence of an independent jalisco block however it is more likely that this misclosure is instead primarily due to the effects of recent changes in the relative motion between the pacific and rivera plates assuming the second explanation is correct a new rivera north american euler pole location 21 8 n 110 4 w is determinable solely from earthquake slip vectors located along the rivera north american boundary

rans based simulation of turbulent wave boundary layer and sheet flow sediment transport processes abstract a numerical model coupling the horizontal component of the incompressible reynolds averaged navier stokes rans equations with two equation k turbulence closure is presented and used to simulate a variety of turbulent wave boundary layer processes the hydrodynamic model is additionally coupled with bed and suspended load descriptions the latter based on an unsteady turbulent diffusion equation for simulation of sheet flow sediment transport processes in addition to standard features common within such rans based approaches the present model includes 1 hindered settling velocities at high suspended sediment concentrations 2 turbulence suppression due to density gradients in the water sand mixture 3 boundary layer streaming due to convective terms and 4 converging diverging effects due to a sloping bed the present model therefore provides a framework for simultaneous inclusion of a number of local factors important within cross shore wave boundary layer and sediment transport dynamics the hydrodynamic model is validated for both hydraulically smooth and rough conditions based on wave friction factor diagrams and boundary layer streaming profiles with the results in excellent agreement with experimental and or previous numerical work the sediment transport model is likewise validated against oscillatory tunnel experiments involving both velocity skewed and acceleration skewed flows as well as against measurements beneath real progressive waves model capabilities are exploited to investigate the importance of boundary layer streaming effects on sediment transport in selected velocity skewed conditions for the medium sand grain conditions considered the model results suggest that streaming effects can enhance onshore sediment transport rates by as much as a factor of two moreover for fine sand conditions streaming and related convective effects are demonstrated to potentially reverse the direction of net transport i e from offshore to onshore relative that predicted in oscillatory tunnel conditions the developed model is implemented within the popular matlab environment and hence may be attractive for both research and educational purposes

seismicity and tectonics of the subducted cocos plate we have examined teleseismic earthquake locations reported by the international seismological centre isc for the middle america region and selected 220 as the most reliable these hypocenters and other data are used to delineate the deep structure of the subducted cocos plate the results indicate that the subducted plate consists of three major segments segment i extends from the panama fracture zone to the nicoya peninsula the structure of this segment is poorly defined segment ii is the largest and best defined segment this segment consists of two parts iia and iib part iia extends from the nicoya peninsula to western guatemala and is very well defined and continuous in structure its strike follows the curvature of the trench and dips at about 60 part iib extends from western guatemala to orizaba mexico the dip of this part of the segment decreases slightly toward the northwest and its strike is more northward than that of the trench segment iii extends from orizaba to the rivera fracture zone and is not well defined due to a lack of earthquake activity beneath about 100 km its orientation differs markedly from segment ii and strikes somewhat more westward than the trench between parts iia and iib of segment ii the subducted plate seems to be continuous bending smoothly to accommodate the change in geometry local network data from costa rica suggest there may be a tear between segments i and ii between segments ii and iii there is a gap in the hypocenters which makes it difficult to define the boundary the change in geometry between these two segments indicates that there may be a tear and two strike slip focal mechanisms in the region support this conclusion we find no convincing evidence supporting the existence of segments smaller than the three described above if there is smaller scale segmentation in the shallow part of the subducting plate the plate must still maintain enough continuity to appear continuous at greater depths there is no evidence for any major tear in the subducted plate associated directly with either the tehuantepec ridge or the orozco fracture zone the shallow subduction at the northwestern end of segment ii may be related to the bouyancy of the tehuantepec ridge the cocos ridge is probably directly responsible for the change in geometry between segments i and ii and may even be slowing or stopping subduction in segment i the structure of the subducted plate in segment ii and the changes in the character of volcanism along the arc can be related to the relative motion of the north american and caribbean plates the present geometry of part iib of segment ii is more consistent with the probable configuration of the trench about 7 ma ago than with the present configuration indicating that the north american plate is overriding the subduction zone appendices 2 3 and 4 are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 009 2 50

los azufres silicic center mexico inference of caldera structural elements from gravity aeromagnetic and geoelectric data los azufres geothermal field is located within a silicic volcanic complex in central mexico the complex is one of the major silicic centers in the trans mexican volcanic belt tmvb pradal and robin 1985 first suggested the existence of the los azufres caldera and ferrari et al 1991 recognized the existence of a collapse structure according to pradal and robin this is a caldera of resurgent type this geophysical study aims to contribute to the knowledge of the structure of the los azufres area gravity aeromagnetic magnetotelluric mt and d c vertical electric resistivity soundings were analyzed results show that los azufres is a very structurally complex setting with relatively thin crust caused by the extensional tectonics characterizing this central sector of the tmvb faults belonging to the e w to ne sw extensional neotectonics and nw se basin and range province systems are observed to affect the geologic units of los azufres according to our study the los azufres geothermal field is located in a structural high located in the middle of a sub circular depression delimited to the north northeast by the santa ines range and to the southwest by the mil cumbres formation the larger depression consists of two narrow deep depressions that correspond to la venta and to the valley of juarez they are separated by the above mentioned structural high these sub depressions are believed to be the sites of a maximum caldera collapse and the structural high is interpreted to be at least in part the caldera s resurgent dome geoelectric structure of the caldera derived from d c resistivity indicates that the brines of the los azufres geothermal system ascend along faults both bounding and internally disrupting the structural high resurgent dome a reasonable correlation is observed between gravity and aeromagnetic data

concordant paleomagnetic directions from the tehuantepec isthmus constraints on the neogene evolution of the north american caribbean plate boundary we report paleomagnetic data for miocene igneous rocks of the southeastern tehuantepec isthmus region mexico thermal and alternating field demagnetization reveal predominantly norma polarity magnetization interpreted as primary trm the overall mean direction dee 355 6 inc 3 y k 36 4 ag5 9 3 n 8 sites is statistically indistinguishable from the expected direction calculated using the north american reference pole this result indicates that significant rotation or displacement of the tehuantepec region relative to the craton has not occurred since 13 ma

concentrations in oscillatory sheet flow for well sorted and graded sands in wave generated sheet flow conditions oscillatory near bed flow is large ripples are washed out and sand is mainly transported in a thin layer a few centimetres thick close to and below the no flow bed level new experiments in the aberdeen oscillatory flow tunnel have produced a comprehensive dataset of transport processes for well sorted and graded sands in large scale sinusoidal and asymmetric oscillatory sheet flow conditions in this paper detailed time and height varying concentration measurements are analysed the range and level of detail in the measurements make it possible to quantitatively analyse concentration behaviour more rigorously than before a new empirical equation is presented which characterises the time dependent concentration profile in the sheet flow layer the equation involves two time dependent parameters erosion depth and reference concentration the dependence of both parameters on flow and bed conditions is analysed new results are presented which make it possible to estimate time dependent erosion depth reference concentration and therefore concentration profile the effects of grading on sheet flow concentrations are analysed by comparing results from the large range of sand beds tested

experimental and numerical study of wave induced backfilling beneath submarine pipelines abstract this paper presents results of complementary experimental and numerical studies involving wave induced backfilling of current generated scour holes beneath submarine pipelines the laboratory experiments are conducted in a wave plus current flume utilizing laser doppler anemometry to measure velocities synchronized flow visualizations using digital image technology along with live bed scour and backfilling measurements each experiment is based on a two stage process 1 initial scour induced by a pure current followed by 2 backfilling induced by pure waves either regular or irregular the time series of scour depths are closely monitored through video recordings systematic analysis of these has resulted in a closed form expression for the backfilling time scale which is demonstrated to be a full order of magnitude greater than the well known time scale of scour with both governed primarily by the shields parameter the developed expression is strictly valid for the current to wave backfilling scenarios considered while likely serving as an upper limit for more general wave induced backfilling circumstances the experiments are complemented by similar backfilling simulations utilizing a fully coupled hydrodynamic and morphodynamic cfd model the numerical simulations demonstrate the ability of the model to predict backfilling towards expected equilibrium scour depths based on the new wave climate with time scales reasonably in line with experimental expectations

measurements of sheet flow transport in acceleration skewed oscillatory flow and comparison with practical formulations near bed oscillatory flows with acceleration skewness are characteristic of steep and breaking waves in shallow water in order to isolate the effects of acceleration skewness on sheet flow sand transport new experiments are carried out in the aberdeen oscillatory flow tunnel the experiments have produced a dataset of net transport rates for full scale oscillatory flows with varying degrees of acceleration skewness and three sand sizes the new data confirm previous research that net transport in acceleration skewed flow is non zero is always in the direction of the largest acceleration and increases with increasing acceleration skewness large transport rates for the fine sand conditions suggest that phase lag effects play an important role in augmenting positive net transport a comparison of the new experimental data with a number of practical sand transport formulations that incorporate acceleration skewness shows that none of the formulations performs well in predicting the measured net transport rates for both the fine and the coarser sands the new experimental data can be used to further develop practical sand transport formulations to better account for acceleration skewness

timing and extent of neogene tectonic rotation in the western transverse ranges california paleomagnetic data collected from lower miocene rocks throughout the western transverse ranges province have consistently clockwise deflected magnetic declinations which suggests that the entire province has experienced clockwise tectonic rotation during neogene time paleomagnetic declinations in lower miocene rocks increase westward from 33 5 c 11 in the san gabriel block to 91 7 7 in the western santa ynez range suggesting a westward increase in the net amount of clockwise tectonic rotation paleomagnetic data that constrain the tuning of rotation suggest that the entire western transverse ranges rotated uniformly 50 60 clockwise during the middle miocene this implies a minimum of 200 km of distributed dextral shear along the california coast and offshore during middle miocene time little tectonic rotation appears to have occurred in the western transverse ranges during late miocene time since miocene time continued clockwise rotation in the western santa ynez range 31 3 combined with counterclockwise rotation in the san gabriel block 16 has oroclinally bent the transverse ranges to the west of the san andreas fault the pliocene pleistocene clockwise rotation implies an additional 30 60 km of distributed dextral shear across the southern california coast ranges during post miocene time

palaeomagnetic results from the southern sierra madre oriental mexico evidence for early cretaceous or laramide remagnetization abstract a large suite of samples from the latest triassic huizachal and early jurassic huayacocotla groups and the latest jurassic taman and earliest cretaceous pimienta formations was collected in the southern sierra madre oriental for a palaeomagnetic study only the samples from three sites belonging to the huizachal group and the las juntas formation possibly have retained their primary magnetization if so their pole position does not reveal any palaeomagnetically discernable motion relative to cratonic north america all remaining sites were remagnetized as evidenced by a negative fold test at four sites and the fact that the pole positions cluster better at the 95 significance level if no structural corrections are applied the tightness of the cluster 95 4 6 and the same polarity suggest that the samples were remagnetized at some common time these results permit two interpretations 1 if the sampling region has not suffered any significant tectonic rotation then the remagnetization can be dated by comparison with the polar wander path for north america as early cretaceous 130 ma this early cretaceous phase of deformation is not recognized in the northern sierra madre oriental and clearly pre dates the early tertiary laramide orogeny 2 if the southern sierra madre oriental did rotate counterclockwise by 20 then the remagnetization could have originated in any cretaceous or early tertiary time and may indeed be related to the laramide deformation in either case the data imply that the southern sierra madre oriental constitutes an independent tectonic domain

experimental study of tsunami induced scour around a monopile foundation abstract this paper presents an experimental study of the tsunami induced scour process around a monopile foundation representative of those commonly used for offshore wind farms the scour process is studied by subjecting the monopile to a time varying current which enables a properly down scaled experiment from the boundary layer and scour perspective it is shown how the scaled experiments corresponds to real life idealized tsunami cases with periods ranging from 10 to 40 min it is then shown that the boundary layers of the model tsunami are well described by recently developed empirical relations for tsunami boundary layers by subjecting the monopile to several successive tsunami waves the scour process is shown to occur in a stepwise cumulative fashion with the final equilibrium scour depth tending to the depth limited steady current limit it is shown that the entire scour development can reasonably be predicted by a recently developed simple engineering model finally the experimental results are compared to a fully coupled hydrodynamic and morphologic cfd model and a good correspondence is obtained

influence of turbulence on bed load sediment transport this paper summarizes the results of an experimental study on the influence of an external turbulence field on the bed load sediment transport in an open channel the external turbulence was generated by 1 a horizontal pipe placed halfway through the depth ih 2 a series of grids with a clearance of about one third of the depth from the bed and extending over a finite length of the flume and 3 a series of grids with a clearance in the range 0 1 1 0 ih from the bed but extending over the entire length of the flume two kinds of experiments were conducted plane bed experiments and ripple covered bed experiments in the former case the flow in the presence of the turbulence generator was adjusted so that the mean bed shear stress was the same as in the case without the turbulence generator in order to single out the effect of the external turbulence on the sediment transport in the ripple covered bed case the mean and turbulence quantities of the streamwise component of the velocity were measured and the shields parameter due to skin friction was determined the shields parameter together with the rms value of the streamwise velocity fluctuations was correlated with the sediment transport rate the sediment transport increases markedly with increasing turbulence level

sediment flux based model of instantaneous sediment transport due to pure velocity skewed oscillatory sheet flow with boundary layer stream abstract velocity skewed oscillatory sheet flow transport is studied by an approximate model for instantaneous sediment transport resulting from exponential profiles of sediment concentration and velocity over a mobile bed the phase lag effect and asymmetry in wave friction factor oscillatory flow orbital amplitude roughness height bed shear stress and wave boundary layer thickness are modelled so that the net boundary layer stream of pure velocity skewed oscillatory flow can be obtained the proposed model is found consistent with the previous classical formulas and qualitatively refines a series of exponents of velocity power function for the approximation of instantaneous sediment transport rate under different flow conditions the sediment fluxes can be well predicted with accuracy as that of a two phase numerical model the net boundary layer stream and sediment transport rates are also well predicted especially the negative net stream and rates with large phase residual the phase residual is shown a useful but insufficient factor in the net sediment transport in pure velocity skewed flows in addition the net boundary layer stream resulting from velocity skewed wave boundary layer development difference between onshore and offshore flow durations is an essential factor as well

net sediment transport under wave groups and bound long waves the net sediment transport under regular waves and under wave groups with and without bound long waves is investigated by mathematical modeling the bed is assumed to be plane sheet flow conditions which occurs for shields parameters larger than about 1 the turbulent boundary layer flow is modeled by a mixing length model the suspended sediment is modeled by the advection diffusion equation the net sediment transport due to streaming mass drift and bound long waves is considered it is found that the bound long waves cause an additional forward motion of the water near the bed the bound long waves further give a negative contribution to the sediment transport because the backward motion in the long waves is coupled with the high waves and high sediment concentrations the combined net sediment transport is found to depend on the grain size and the transport intensity for very fine sand the mean water motion and the forward drift of the sediment dominate giving a positive net transport for coarser sand the coupling between backward motion and high sediment concentration dominates giving a negative contribution a parameter study is carried out to investigate the dependence of the net sediment transport on variations of the dimensionless parameters that characterize the condition

comparisons between sediment transport models and observations made in wave and current flows above plane beds as a part of the mast2 g8 m coastal morphodynamics project the predictions of four sediment transport models have been compared with detailed laboratory data sets obtained in the bottom boundary layer beneath regular waves asymmetrical waves and regular waves superimposed co linearly on a current each data set was obtained in plane bed sheet flow conditions and each of the four untuned numerical models has provided a one dimensional vertical 1dv time varying representation of the various experimental situations comparisons have been made between the model predictions and measurements of both time dependent sediment concentration and also wave averaged horizontal velocity and concentration for the asymmetrical waves and for the combined wave current flows comparisons have been made with vertical profiles of the cycle averaged sediment flux and also with the vertically integrated net sediment transport rate each of the turbulence diffusion models gives an accurate estimate of the net transport rate invariably well within a factor of 2 of the measured value in contrast none of the models provides a good detailed description of the time dependent suspended sediment concentration due mainly to the inability of conventional turbulence diffusion schemes to represent the entrainment of sediment into suspension by convective events at flow reversal however in the cases considered here this has not seriously affected the model predictions of the net sediment flux due to the dominance of the near bed transport the comparisons in this paper are aimed not only at testing the predictive capability of existing sediment transport modelling schemes but also at highlighting some of their deficiencies

the evolution of middle america and the gulf of mexico caribbean sea region during mesozoic time a plate tectonic model for the evolution of middle america and the gulf of mexico caribbean sea region is presented the model which is based upon the existence of the mojave sonora megashear incorporates into the triassic pangea reconstruction three microplates between north and south america thus avoiding the overlap of the bullard fit these plates are the yaqui bounded on the north by the mojave sonora megashear the east and west maya plates bounded on the north by the mexican volcanic zone and on the south by a predecessor of the motagua fault zone and the chortis plate parts of guatemala and honduras during late jurassic time as north america split away from europe africa and south america shear with left lateral sense of displacement occurred along the transform faults that bounded the micro plates if 800 km of left lateral displacement along the mojave sonora megashear 300 km along the mexican volcanic belt and 1 300 km along a proto motagua megashear are restored and if yucatan and cuba are rotated to fit against northern south america then 1 a curvilinear belt of late paleozoic rocks that show lithologic as well as paleontologic similarities extends across the reconstruction and links outcrops in texas eastern mexico nuclear central america and colombia 2 a mediterranean like sea is delineated that was a precursor of most of the present gulf of mexico 3 correlation is implied between the distinctive quartzose san cayetano formation of cuba and the caracas and juan griego groups of venezuela geometric constraints suggest that probably shear initially occurred along the mexican volcanic zone near the end of the middle jurassic subsequently probably about 160 m y ago displacements that total 800 km began along the mojave sonora megashear contemporaneously yucatan and fragments of pre cretaceous rocks that compose parts of central and western cuba migrated northward toward their present positions rotation of yucatan was facilitated by considerable displacement along the proto motagua zone and along a zone that is probably coincident with the modern salina cruz fault accumulation of widespread major salt units of late jurassic callovian to early oxfordian age in the gulf basin probably occurred contemporaneously with the arrival of these blocks at their present positions clastic units that interfinger with some of the youngest salt units and rim the gulf of mexico have not recorded major recognized translations since their accumulation clockwise rotation of south america and the chortis plate occurred during early cretaceous time this movement which was manifested by subduction of jurassic ocean floor against the previously rifted precursor of the island of cuba and under parts of hispaniola and puerto rico is recorded by circum caribbean orogeny abrupt changes in the relative motions between north and south america during late cretaceous time may have resulted in extension and outpourings of basalt upon the jurassic rocks of the ocean floor of the venezuelan basin west of beata ridge sea floor spreading formed the colombian basin related subduction occurred as the chortis plate including part of central america the nicaraguan rise and southeastern cuba was sutured against the maya east plate along the present motagua fault and cayman trench our model is constrained by published geologic data the relative positions of north and south america from atlantic sea floor magnetic anomalies and the requirement that the major transform faults be compatible with the poles of rotation for the appropriate relative motions between north and south america paleomagnetic data from middle america are sparse but do not conflict with the predicted motions of some of the microplates especially chortis

lateral and temporal changes in volcanogenic sedimentation analysis of two eocene sedimentary aprons big bend region texas abstract middle to late eocene volcanic activity in the big bend region of texas induced dramatic changes in local depositional processes recorded by volcanogenic sequences in the chisos canoe and devil s graveyard formations analysis of lithofacies associations clast lithologies and paleocurrent trends of tuffaceous strata in these sequences allows recognition of two sedimentary aprons derived from two widely spaced volcanic centers a southern apron shed from a center in mexico and an apron shed from the christmas mountains volcanic center each volcanogenic apron contains distinctive vertical sequences that reflect in different ways episodic explosive activity at the volcanic source area and also distinctive records of lateral changes in sedimentation that correspond to distance from the volcanic source area the southern apron contains intermediate source deposits that are the product of alternating modes of deposition that correspond to aggradation during explosive volcanic activity as well as aggradation during volcanic quiescence mudflow and hyperconcentrated flood flow deposition occurred during times of explosive activity and fluvial deposition and pedogenesis occurred during volcanic quiescence stacked beds of lithic clast deficient mudflow deposits are the result of rapid aggradation of intrabasinal derived flows on a topography inundated with tephra distal source deposits do not show evidence for alternating modes of deposition but instead were deposited almost entirely by traction and suspension processes in a fluvial system and subsequently modified by pedogenesis eruptions associated with the christmas mountains caldera complex at 42 ma produced abundant lava and pyroclastic deposits that are interstratified with sedimentary sequences proximal source exposures within 8 km of the calderas consist mostly of ashflow and air fall tuffs and lava and debris flow deposits occurring as five distinctive sequences that reflect the episodic eruptive history at this center intermediate source exposures from 18 to 23 km are predominantly composed of interbedded mudflow and hyperconcentrated flood flow deposits that are separated by disconformities alternating aggradation and degradation apparently resulted from drastically fluctuating sediment loads originating from the episodically active volcanic source at distal source exposures 30 to 35 km from he caldera there are no apparent disconformities in the apron initial aggradation was dominated by mudflows and pumice fall followed by traction deposition in sheet floods and channels evidence independent of the volcanogenic sequences indicates that each apron aggraded under roughly the same tectonic and climatic influences hence this study shows that volcanogenic sequences can record the explosive activity of a source center in 2 different ways in the same climatic and tectonic conditions in addition this study shows that this record of explosive activity may not be recorded at distal parts of the aprons

toward modeling turbulent suspension of sand in the nearshore 1 we present two depth and phase resolving models based on single and two phase approaches for suspended sediment transport under water waves both models are the extension of a wave hydrodynamic model cornell breaking wave and structure cobras in the two phase approach dilute two phase mass and momentum equations are calculated along with a fluid turbulence closure based on balance equations for the fluid turbulence kinetic energy k f and its dissipation rate f in the single phase approach the fluid flow is described by the reynolds averaged navier stokes equations while the sediment concentration is calculated by an advection diffusion equation for the conservation of sediment mass the fluid turbulence is calculated by k f f equations that incorporate the essential influence of sediment which can also be consistently deduced from the two phase theory by adopting a commonly used sediment flux boundary condition near the bed the proposed models are tested against laboratory measurements of suspended sediment under nonbreaking skewed water waves and shoaling broken waves although the models predict wave averaged sediment concentrations reasonably well the corresponding time histories of instantaneous sediment concentration are less accurate we demonstrate that this is due to the uncertainties in the near bed sediment boundary conditions in addition we show that under breaking waves the near bed sediment pickup cannot be solely parameterized by the bottom friction suggesting that other effects may also influence the near bed sediment boundary conditions

flow tunnel measurements of velocities and sand flux in oscillatory sheet flow for well sorted and graded sands oscillatory flow tunnel measurements of velocities and fluxes for sands in oscillatory sheet flow conditions are presented the experiments involved a range of well sorted and graded sands in two asymmetric flows velocities were measured using an ultrasonic velocity profiler uvp capable of measuring deep within the sheet flow layer velocity profiles are found to be similar for the same flow but different sand beds and display expected features of oscillatory boundary layer flow the near bed velocity leads the main flow velocity by approximately 21 and a small offshore directed current is generated near the bed measures of the boundary layer thickness are in good agreement with those predicted using an equation formulated for the boundary layer thickness over fixed beds velocity data have been combined with concentration data to produce time dependent sand flux profiles covering the sheet flow and suspension regions there are fundamental differences in the transport processes of sands of different size and grading caused by unsteady effects which dominate in the case of fine sand and are largely absent in the case of coarse sand 1 time averaged flux is onshore directed positive in the case of coarse sand and is confined to a region immediately above the bed in contrast time averaged flux in the case of fine sand extends high above the bed is offshore directed negative in the sheet flow layer and becomes onshore directed in the suspension layer 2 net transport in the case of coarse sand is directed onshore as the percentage of fine sand in the bed increases offshore transport becomes increasingly dominant as the percentage of fine sand increases 3 net transport in the suspension layer is onshore while net transport in the sheet flow layer may be onshore or offshore depending on sand size and grading

paleomagnetic and magnetic fabric studies of the san gaspar ignimbrite western mexico constraints on emplacement mode and source vents abstract paleomagnetic and magnetic fabric data for the 4 8 ma san gaspar ignimbrite one of the largest in western mexico are used to investigate the source vents emplacement mechanism and tectonics rock magnetic properties from distant parts of the ignimbrite are similar suggesting relatively homogeneous mineralogy of the unit isothermal remanence and continuous susceptibility temperature experiments point to low to medium ti titanomagnetites as the main magnetic carriers hysteresis ratio parameters of most samples fall in the pseudo single domain grain size region wasp waisted hysteresis loops were identified corresponding to high hcr hc values stepwise thermal and alternating field demagnetization shows that secondary components are completely removed below 20 mt or 400 c thereafter the characteristic component is isolated with small within site dispersion of mean direction 95

experimental study of sheet flow sediment transport under nonlinear oscillatory flow over a sloping bed abstract the seabed is sloped in shallow coastal regions where strong nonlinear shoaling waves can generate sheet flow sediment transport both wave nonlinearity and bottom slope can lead to a wave averaged net sediment transport rate but the bottom slope effect is often overlooked in existing sediment transport models a full scale experimental study of sheet flow sediment transport under the oscillatory flows with nonlinear features skewness or asymmetry was conducted in an inclinable oscillatory water tunnel so the bottom slope induced net transport rate in the context of nonlinear waves was experimentally investigated the tests cover a variety of nonlinear wave shape bottom slope 0 to 2 5 and two sediment diameters 0 24 and 0 51 mm since the downslope direction is the offshore direction in our tests it is found that bottom slope reduces the net onshore transport rate due to wave nonlinearity the slope contribution to the net transport rate is evaluated by taking the difference between the measured net transport rate from a sloping bed experiment and the measured net transport rate from the corresponding horizontal bed experiment the results suggest that slope induced net transport rate is insensitive to wave nonlinearity and its variation with bottom slope can be considered linear in the context of predicting net transport rate comparisons between this study and a preceding sinusoidal flow study of yuan et al 2017b suggest that nonlinear waves can be approximated as sinusoidal flows for estimating the slope contribution to the net transport rate subsequently an empirical model for predicting net sheet flow transport rate due to bottom slope is established based on dimensional analysis this model can be simply added to existing sheet flow sediment transport models to improve their applicability in coastal regions

numerical simulation of wave induced scour and backfilling processes beneath submarine pipelines abstract a fully coupled hydrodynamic morphodynamic numerical model is presented and utilized for the simulation of wave induced scour and backfilling processes beneath submarine pipelines the model is based on solutions to reynolds averaged navier stokes equations coupled with k turbulence closure with additional bed and suspended load descriptions forming the basis for sea bed morphology the morphological evolution is updated continuously rather than being based e g on period or other time averaging techniques simulations involving wave induced scour over the range of keulegan carpenter number 5 6 kc 30 demonstrate reasonable match with previous experiments both in terms of the equilibrium scour depth as well as the scour time scale wave induced backfilling processes are additionally studied by subjecting initial conditions taken from scour simulations with larger kc to new wave climates characterized by lower kc values the simulations considered demonstrate the ability of the model to predict backfilling toward expected equilibrium scour depths based on the new wave climate in line with experimental expectations the simulated backfilling process is characterized by two stages 1 an initial re distribution phase involving re organization of sediments in the immediate vicinity of the pipeline potentially followed by 2 a more lengthy backfilling evolution toward equilibrium scour depth the simulated backfilling time scales are of the same order of magnitude as in experiments though the multi stage process complicates a more systematic characterization the simulated sequences of scour and backfilling achieved within the present work are estimated to represent temporal durations of up to approximately 12 h at full practical scales

data analysis of bed concentration of suspended sediment an empirical relation for the bed concentration of suspended sediment cb defined at a few grain diameters above the bed is developed its derivation is based on the experimental data sets of guy et al in 1966 the values of the near bed concentration were obtained by first isolating the suspended load transport from the observed rates of total load transport by use of a bed load formula the suspended load was then equated to einstein s formulation which allowed the determination of cb the influence of large concentrations of suspended sediment and of the reference level for cb on the calculated values of bed concentration has been analyzed the proposed relation compares satisfactorily to values of near bed concentration obtained from independent data sets the procedure used to derive cb provides a simple method for the calculation of the total transport rate of noncohesive sediment

paleomagnetic study of the eastern sector of chapala lake and implications for the tectonics of west central mexico abstract western mexico is characterized by several large scale tectonic depressions which have been interpreted in terms of active continental rifting tectonic transpression and coastal sinistral lateral transport of terranes in this paper we report results of a paleomagnetic study of 148 samples from 22 sites in the neogene volcanics from the eastern sector of the chapala graben western end of the e w chapala tula fault zone characteristic remanent magnetization directions have been isolated after detailed thermal demagnetization for sixteen sites six sites present reverse polarities and ten sites present normal polarities two sites show a low latitude vgp and are considered as transitional the normal and reverse polarity directions are almost antipodal the overall characteristic pole position for the normal and reverse polarity sites n 16 p lat 74 n p long 160 e k 25 and a 95 7 6 lies to the left of the neogene segment of the north american or northern mexico apparent polar wander path which suggest the occurrence of a counterclockwise rotation of 15 5 7 4 to 16 5 6 9 results are interpreted in terms of counterclockwise vertical axis rotation associated with regional transtension and left lateral shear as a result of oblique subduction of the cocos plate along the middle american trench we suggest that the chapala graben developed early in the miocene in a left lateral strike slip environment within the regional e w chapala tula fault zone results support recent studies that propose relative motion for southern mexico along the volcanic arc and trench parallel strike slip faulting

normal amplitude brunhes paleosecular variation at low latitudes a paleomagnetic record from the trans mexican volcanic belt ancient secular variation in central mexico was determined from paleomagnetic measurements on 45 independent lava flows ranging in age 14c and k ar dates from 2 500 to 580 000 years b p all the analyzed flows are characterized by a normal polarity placing them within the brunhes chron the paleosecular variation analysis yielded an angular standard deviation from the field of an axial dipole of 15 3 with 95 confidence limits of 13 4 and 17 9 and that of the corresponding virtual geomagnetic poles vgps of 13 9 with confidence limits of 12 1 and 16 1 these values are in agreement with those predicted by most statistical models of secular variation but divergent from those obtained from low latitudes 19 n such as hawaii and pagan is

tectonics and volcanism of sierra chichinautzin extension at the front of the central trans mexican volcanic belt abstract because of its recent activity and position at the southern magmatic front of the trans mexican volcanic belt tmvb the sierra chichinautzin volcanic field scn is a key area for the understanding of this controversial volcanic province volcanic activity has built more than 220 monogenetic volcanoes shields scoria cones thick lava flows and hydromagmatic structures during the last 40 000 years for a total volume of about 470 km 3 the scn basalts are geochemically similar to oibs while the intermediate and felsic volcanic rocks show a calc alkaline trend and abundant evidence for magma mixing the structural analysis of this volcanic field and surrounding areas has been based on field data satellite images and a method for detecting volcanic center alignments the tectonic data together with geophysical evidence confirm active general n s extensional conditions with a strike slip component for the scn area the same structural setting that prevails in the rest of the central tmvb extensional tectonics a negative regional bouger gravity anomaly a low velocity mantle high heat flow and shallow seismicity suggest a rift type setting involving the upwelling of anomalous mantle beneath the central tmvb the combined petrological structural and geophysical arguments support that the scn volcanism is rift related and rule out processes involving the subduction of the cocos plate which casts further doubts on the standard subduction model for the tmvb volcanism