fish canyon tuff colorado the problem of two magnetic polarities in a single tuff abstract the fish canyon tuff located in the central san juan mountains southern colorado is currently recognized as one of the largest ash flow tuffs in the world paleomagnetic samples from 21 sites have been obtained from a composite vertical section of the tuff the two uppermost sites from the otherwise normally magnetized tuff exhibit stable but anitpodal reversed remanent magnetic rm directions field relationships and geochemical consistencies indicate that the entire section was emplaced relatively rapidly probably in

post oligocene river incision southern sierra madre occidental mexico the sierra madre occidental of western mexico consists of a granitic basement covered by oligocene ignimbrites that define a reference surface from which to estimate late cenozoic river incision a 90 m grid digital elevation model was used to characterize contemporary topography and interpolate the late oligocene surface of the ignimbrite plateau from a surface fit to the highest points in the relatively undissected uplands between major river valleys long term river incision rates calculated from the difference between this reference surface and longitudinal profiles of 11 rivers that flow toward the tepic zacoalco rift zone range from about 0 01 to 0 2 mm year 1 river profiles of this region also show evidence of river capture driven by flexural uplift along the flank of the rift zone river profile concavity values in the sierra madre occidental range from 0 22 to 0 63 a range similar to that reported previously for a wide range of environments in contrast the empirically constrained ratio of exponents in the stream power model of river incision m n ranges from 0 44 to 0 52 close to the expected theoretical value of 0 5 the wider range of observed values may illustrate how can differ from the driving values of m n in non steady state bedrock river systems

emplacement of pyroclastic dykes in riedel shear fractures an example from the sierra de san miguelito central mexico abstract although magmatic dykes are considered to be emplaced at depth there are very few studies about the emplacement of pyroclastic dykes there are two commonly accepted mechanisms for dike emplacements namely formation of a hydraulic tension fracture and filling of pre existing fractures in the sierra de san miguelito central mexico there is a dike swarm with orientations that do not concur with any of the above two cases instead the dikes have orientations resembling some type of secondary fractures within the fault bounded blocks within a fault bounded block the reverse drag of the normal faults can be explained by simple shear based on the simple shear mechanism we establish a series of equations to calculate the extension and the direction of the simple shear within a fault bounded block as an application of our methodology the value of which is the angle measured from the vertical to the direction of the simple shear is calculated from the domino faults in the sierra de san miguelito mesa central mexico the results demonstrate that the absolute values of for the inclined shear are smaller than 34 the inclined shear can be antithetic 0 synthetic 0 the pyroclastic dykes in the study area have most of the strikes between 300 and 330 and are sub parallel to the major faults the preferred dips of the pyroclastic dykes vary from 80 to 90 the distribution of the pyroclastic dykes in the study area indicates that the dikes were primarily emplaced along the r fractures due to simple shear these results are different from the traditional understanding which assumes that the dykes were mainly emplaced within the tension fractures

tectono stratigraphic evolution of the comondu group from bahia de la paz to loreto baja california sur mexico abstract the late oligocene to mid miocene volcanic and volcaniclastic rocks of the comondu group are well exposed along the main rift escarpment of baja california sur from the bahia de la paz region to bahia concepcion new mapping and stratigraphic analysis of the comondu group from bahia de la paz to loreto reveal facies trends and correlations that form the foundation for a continuous stratigraphic framework for the comondu group along a 300 km long transect on the eastern coast of the baja california peninsula broad but distinct lithostratigraphic trends alluvial fan facies and volcanic and volcaniclastic facies record an overall coarsening upwards package that includes ignimbrite deposits within increasingly proximal alluvial fan deposits both derived from the east geochronology of the unit including 32 isotope ages and 12 previously unpublished 40ar 39ar ages provide the timing of four main increasingly proximal depositional events non marine sandstone defining the base of the comondu group was first deposited between 26 ma and 24 ma emplacement of rhyolitic ignimbrites initiated between 24 ma and 23 ma and marked a westward expansion of volcanic activity affiliated with the sierra madre occidental ignimbrite sequences in southern sinaloa western durango and northern nayarit a change in volcanism occurred at 19 ma to 18 ma with more ignimbrites increased intermediate compositions and the appearance of local vents and proximal volcanic facies a final localized change of volcanism occurred from 14 to 12 ma in the loreto area with an increase of proximal alluvial fan deposits and local volcanoes in the upper comondu group the bulk of the upper comondu group is absent south of the loreto area and has either been removed by erosion as a source for the magdalena fan in the pacific ocean or was focused primarily in the loreto area and northward we use a pre rift tectonic reconstruction of the gulf of california to align broad stratigraphic trends along the peninsula the gulf conjugate margins and within the gulf the cascadia arc of oregon and northern california may be a modern analog for the comondu group with a linear volcanic arc formed above shrinking subducting microplates and a broad backarc region of moderate extension and scattered volcanism

reconciling tectonic shortening sedimentation and spatial patterns of erosion from 10 be paleo erosion rates in the argentine precordillera abstract the temporal evolution of erosion over million year timescales is key to understand the development of mountain ranges and adjacent fold and thrust belts while models of orogenic wedge dynamics predict an instantaneous response of erosion to pulses of rock uplift stream power based models predict that catchment wide erosion maxima significantly lag behind a pulse of rock uplift here we explore the relationships between rock uplift erosion and sediment deposition in the argentine precordillera fold and thrust belt at 30 s using a combination of 10be derived paleo erosion rates constraints on re exposure using 26al 10be ratios geomorphic observations and detrital zircon provenance we demonstrate that the attainment of maximum upland erosion rates lags the maximum rate of deformation over million year timescales the magnitudes and causes of the erosional delays shed new light on the catchment erosional response to tectonic deformation and rock uplift in orogenic wedges

nature and timing of faulting and synextensional magmatism in the southern basin and range central eastern durango mexico the nazas area in the central eastern portion of the state of durango is within the poorly studied southern half of the basin and range province in mexico at nazas high angle normal faults that cut part of the mid tertiary volcanic sequence strike between n 20 and 70 w with 40 of them between n 40 and 50 w tilting of faulted blocks varies from 5 to 35 to the northeast most commonly around 25 ne a few blocks are tilted to the southwest as much as 25 fault offsets range from 40 m to nearly 300 m the earliest faulting occurred between 31 and 29 ma before the emplacement of mid tertiary ash flow tuffs had ended the santa ines formation a widespread fanglomerate was deposited after the earliest faulting episode and is overlain by 24 to 20 ma alkalic basalt flows although not cut by faults the flows are adjacent to or cover normal faults some of the mapped faults could have been coeval with basalt eruption as is the case in trans pecos texas where alkalic basalt having similar age and composition to that in the nazas area erupted contemporaneously with normal faulting the nazas alkalic basalt also has similar age and stratigraphic position as but is compositionally distinct from the southern cordilleran basaltic andesite scorba a widespread mafic suite in southwestern north america that has been linked to regional extension

the sierra de mil cumbres michoacan mexico transitional volcanism between the sierra madre occidental and the trans mexican volcanic belt abstract the sierra de mil cumbres is a miocene volcanic range located in central mexico in the north eastern part of the state of michoacan near the city of morelia structurally it is a ene trending horst that covers an area of 1022 km2 approximately 20 km wide 60 km long and contains exposures of chemically bimodal volcanism in the form of ignimbrites lava domes lava flows cinder cones and related deposits the main volcanic manifestations of this range are the la escalera caldera 16 3 23 ma the garnica volcanic complex 18 3 17 9 ma the atecuaro caldera 16 3 19 4 ma and the indaparapeo volcanic complex 14 1 17 5 ma the sierra de mil cumbres stands in space and time at the intersection between the miocene recent trans mexican volcanic belt and the late cretaceous early miocene sierra madre occidental and so provides new insights into the geological evolution of central mexico arc volcanism in the sierra de mil cumbres was initiated by a massive nnw sse extension probably during the counterclockwise rotation of the sierra madre occidental new geological mapping stratigraphic analysis detailed geochemistry and 40ar 39ar geochronology demonstrates that this intra plate volcanism was emplaced between 14 and 23 ma

evolution of waning subduction related magmatism northern sierra madre occidental mexico the stratigraphic record at the tomochic volcanic center chihuahua documents a change from andesitic to rhyolite dominated volcanism with late extrusion of mafic lavas during the final 10 m y of subduction related activity in the northern sierra madre occidental andesitic volcanism which started before 38 ma and continued until 35 ma was followed by eruption of large volume rhyolite ash flows from the las varas 34 1 ma and tomochic 31 8 31 4 ma calderas basaltic andesite the most mafic rock type in the area was extruded near the end of calcalkalic volcanism at 30 ma the change in style of volcanism at the tomochic volcanic center also occurred in other parts of the northern sierra madre occidental and is interpreted to reflect the evolution of waning subduction related magmatism as convergence between the north american and farallon plates slowed before it eventually ceased slowed plate convergence resulted in a decreased flux of mafic melts from the mantle to the crust leading in turn to the stagnation and coalescence of intermediate composition magmas similar to those that fed early andesitic eruptions subsequent differentiation led to growth of zoned magma reservoirs from which the rhyolite ash flows were extruded slowed convergence was accompanied by the onset of crustal extension which possibly aided the coalescence of andesitic magmas and which allowed the ascent and eruption of mafic melts near the end of silicic volcanic activity although minor differences exist the history of mid tertiary magmatism and tectonism in the northern sierra madre occidental was broadly similar to that in parts of the south western united states

guerrero terrane of mexico its role in the southern cordillera from new geochemical data the guerrero terrane makes up most of the western part of mexico is one of the largest terranes of the north american cordillera and is characterized by an upper jurassic lower cretaceous volcanic sedimentary sequence of are affinity metamorphic rocks that crop out in the western area of the terrane arteaga complex may represent its basement they are mostly composed of terrigenous sediments varales formation with minor basaltic pillow lavas chert tuff and limestone initial nd values 13 and rare earth element ree values for pillow lavas of the arteaga complex are characteristic of mid ocean ridge basalts morb in contrast the varales formation sedimentary rocks from the arteaga complex have negative initial nd 6 2 and 7 2 and are enriched in light rees these data indicate that the sediments of the varales formation were supplied from an evolved continental crust the overlying jurassic cretaceous arc related rocks have initial nd 7 9 to 3 9 and ree patterns similar to those of evolved intraoceanic island arcs these data show that the evolution of the guerrero terrane had an early pre cretaceous stage which consisted of an oceanic crust receiving sediments from a continental source and a cretaceous stage which was the development of an island arc the oceanic continental isotopic signature of the arteaga complex is different from other western north american cordilleran terranes e g alexander wrangelfia that are more completely oceanic in affinity nevertheless the extensive jurassic cretaceous arc represents additions of juvenile material to the western north american cordillera

thermal history of a bishop tuff section as determined from the width of cryptoperthite lamellae the lamellar spacings in the sanidine cryptoperthites bulk composition or 66 4 ab 32 3 an 1 3 from the bishop tuff near long valley california have been determined using transmission electron microscopy the lamellar spacings show a sharp maximum at about 43 m below the topographically highest sample and a weak minimum about 4 m above this this pattern indicates that two flows compose the section the gradational boundary between the flows is about 37 m below the highest sample heat flow calculations applied to the pattern of lamellar spacings indicate that these flows were emplaced within 1 5 yr of each other the result illustrates the power of this technique in deciphering the geologic history of igneous rocks

large scale stable isotope alteration around the hydrothermal carbonate replacement cinco de mayo zn ag deposit mexico carbonate hosted hydrothermal deposits typically show narrow visible mineralogical and textural alteration halos which inhibit exploration targeting in contrast hydrothermal modification of the country rock s stable isotope composition usually extends far beyond the limited visible alteration hence stable isotope studies should be an effective tool to aid exploration for carbonate hosted deposits here we present new insight into the development of a large stable isotope alteration halo based on 910 o and c isotope analyses of carbonate veins and hydrothermally altered limestone hosting the cinco de mayo pb zn ag au cu carbonate replacement deposit crd in chihuahua mexico our results demonstrate that stable isotope alteration is consistent with reactive magmatic fluid flow into unaltered limestone and represents a powerful tool for the characterization of these hydrothermal ore systems synmineralization veins are texturally and isotopically distinct from those formed during pre and postmineralization diagenesis and fluid flow and show distinct gradients along the direction of mineralizing fluid flow this appears to be a promising exploration vectoring tool downhole variations in wall rock isotope values reveal aquifers and aquicludes and outline the principal hydrothermal flow paths furthermore wall rock 18ovsmow systematically decreases toward mineralization from 23 to 17 over a distance of 10 km providing another vectoring tool the extent of the stable isotope alteration halo likely reflects the overall fluid volume and areal extent of a fossil hydrothermal system which may be expected to scale with the mineral endowment this suggests that constraining the size shape and degree of isotopic alteration has direct application to mineral exploration by outlining the system and indicating the potential size of a deposit

late cenozoic basin and range structure in western mexico adjacent to the gulf of california the area that surrounds the gulf of california was intensely faulted during the late cenozoic prior to opening of the gulf in a representative 10 000 km 2 area of southern sinaloa the faults consist of a predominant north northwest striking set having normal displacement and a subsidiary east northeast set having largely strike slip displacement two domains are recognized one in which both fault trends are abundant and one in which east northeast faults are minor in the former north northwest faults form a series of mostly southwest tilted half grabens filled with upper tertiary sediments displacement on individual faults is commonly as much as several kilometers east northeast faults probably represent accommodation zones between areas of differential extension in the second domain north northwest faults form an extensive graben system major faults dipping 40 to 70 into the graben and having several kilometers of cumulative displacement are spaced every 5 to 10 km bedding attitudes indicate that fault blocks are rotated as much as 65 dependent upon assumptions about subsurface geometry of the faults total extension may range from 20 to 50 fault geometry and stress orientations calculated from fault slip data indicate that the least principal stress was east northeast k ar ages of three north northwest striking dikes indicate that east northeast extension began as early as 32 ma ages of tilted volcanic rocks however indicate that measurable tilting and therefore most faulting began after about 17 ma faulting having similar timing style and orientation occurred throughout the area surrounding the gulf of california as far south as nayarit on the mexican mainland and as far north as areas in sonora that are unequivocally within the basin and range province this continuity and similarity in characteristics of faulting around the gulf to those of early basin and range faulting in the united states indicate that the area around the gulf is part of the basin and range province this basin and range faulting probably created the widely recognized but debated proto gulf of california the present gulf of california opened by ocean floor spreading and transform faulting in a zone already weakened by basin and range extension the lack of correspondence between areas of extension and likely areas of crustal thickening related either to laramide contraction or magmatism argues against a model of basin and range extension related solely to spreading of overthickened crust

basaltic volcanism and extension near the intersection of the sierra madre volcanic province and the mexican volcanic belt three groups miocene pliocene and pleistocene off oceanic type basalts associated with normal faulting have been identified in an area surrounding the city of guadalajara mexico although most basalts within these groups have compositional characteristics of an asthenospheric source each group is also associated with lavas that have subduction related traits the first group the san cristobal plateau basalts has a minimum volume of 1 800 km 3 of predominantly alkali olivine basalt and lesser basaltic andesite and fills a pre existing extensional basin of unknown configuration 10 m y ago the relatively rapid eruption of these basalts along with their volume structural association and chemistry suggest an upwelling mantle plume beneath the guadalajara region this region may be the earliest indication of the separation of the jalisco block from north america the second basalt group the guadalajara basalts consists of small volumes of porphyritic basalts and basaltic andesites 3 3 5 0 ma in age that outcrop near the northern outskirts of guadalajara and near the town of hostotip aquillo to the northwest the youngest basalts in the guadalajara area are the 0 4 1 4 ma santa rosa suite of basalts hawaiites mugearites close to the contemporary 0 2 ma andesitic central volcano v tequila these either flowed from nearby cinder cones toward the rio santiago canyon or erupted in the canyon to dam the river the flat lying tertiary 23 27 ma ash flows of the sierra madre volcanic province that lie to the north of guadalajara are faulted north northeast into horsts and grabens that in one case can be dated as being older than 21 8 ma this fault trend has been reactivated closer to guadalajara as normal faults 200 to 300 m displacement cut the miocene plateau basalts but are hidden by a cover of younger volcanic rocks close to the city also hidden by the young volcanic succession there is the north west southeast fault zone that extends to the gulf of california as the tepic zacoalco graben system faulting of this zone is well displayed in the region of the santa rosa dam in the santiago canyon northwest of guadalajara two styles of faulting are found there an older 1 ma 3 has been approximately north northeast for at least 4 m y characteristic of the mexican basin and range province

tectonic implications of post 30 ma pacific and north american relative plate motions the pacific plate moved northwest relative to north america since 42 ma the rapid half rate of pacific farallon spreading allowed the ridge to approach the continent at about 29 ma extinct spreading ridges that occur offshore along 65 of the margin lonsdale 1991 document that fragments of the subducted farallon slab became captured by the pacific plate and assumed its motion prior to the actual subduction of the spreading ridge this plate capture process can be used to explain much of the post 29 ma cordilleran north america extension strike slip and the inland jump of oceanic spreading in the gulf of california the pacific and north american contact zone lengthened with each successive plate capture event underpinning the parts of western north america directly inland with a strong plate undergoing pacific relative motion we suggest that much of the post 29 ma continental tectonism is the result of the strong traction imposed on the deep part of the continental crust by the gently inclined slab of subducted oceanic lithosphere as it moved to the northwest relative to the overlying continent the plate capture hypothesis is distinctly different from theories involving shallow slab gaps kinematic problems associated with shallow slab gap models cause us to question them this conclusion is consistent with seismic refraction interpretations that suggest there is an inclined layer with high velocities like that of basalt or gabbro at the base of the continental crust beneath much of the californian margin and the documented reduction of slab pull forces and density associated with young subducting slabs thermal and rheologic modeling suggests that coastal california was a strong zone at all depths allowing it to be firmly linked to pacific motion our model shows that deformed regions such as the basin and range and borderland provinces developed in predicted weak parts of the crustal section but they have been incompletely linked to the deep plate across the ductile middle and lower crustal layer

the zacatecas mining district a tertiary caldera complex associated with precious and base metal mineralization the zacatecas mining district is located in north central mexico at the transition of the sierra madre occidental and mesa central physiographic provinces the existence of a deeply eroded caldera is evidenced by concentric and radial fracture systems a ring fracture zone a thick pile of ash flow tuffs several subvolcanic plugs a lava flow dome two partially superimposed asymmetric collapse features and a resurgent dome the prevolcanic floor of the caldera is represented by a late cretaceous microdiorite laccolith which intruded metasediments of possible triassic age all overlain by a continental red conglomerate of paleocene age the volcanic center is spatially associated with economic mineralization and exerted strong structural control on the formation of the deposits three groups of veins are recognized varying in ore assemblage and structural position 1 the most important and extensive is the ag pb zn cu au system occurring in concentric and tangential faults and fractures that cut precaldera rocks 2 the au ag se system located in radial fractures that were emplaced in the caldera fill host rocks and 3 the f system subparallel to the other margin of the caldera and also located in caldera fill lithologies silver the dominant precious metal occurs mainly as acanthite silver sulfantimonides such as freibergite and native silver free gold and electrum were identified optically and aguilarite ag 2 ses was identified for the first time in the district pyrite sphalerite galena and chalcopyrite are also present gangue minerals are mainly quartz and calcite preliminary fluid inclusion data on samples from the cantera vein system which contains precious and base metals fall in the temperature range of 300 degrees to 160 degrees c the early tertiary mineralization in the district is epithermal and controlled by volcanotectonic features modified by erosion oxidation and supergene enrichment

variation of cenozoic extension and volcanism across the southern sierra madre occidental volcanic province mexico the middle to late cenozoic tectonic magmatic evolution of the sierra madre occidental volcanic province south of the tropic of cancer is summarized and analyzed for the first time based on new geologic and structural work and published information in the eastern part of the study region mesa central physiographic province silicic volcanism occurred in a short lived episode culminating at ca 30 ma and was followed by crustal scale extension between 30 and 27 ma in the western part of the study area sierra madre occidental physiographic province a voluminous episode of ignimbrite volcanism at 24 21 ma was succeeded by east west extension that produced regularly spaced grabens affecting only the upper crust in the westernmost part of the study region an andesitic to rhyolitic arc formed between 17 and 12 ma was affected by crustal scale north northwest trending extensional faulting leading to the formation of the gulf of california in the mesa central the maximum extension was oriented approximately east west and amounted to 20 in the eastern sierra madre occidental physiographic province extension was only 8 and oriented approximately east west we observe that trenchward shifting of the climax of subduction volcanism and extension occurred during late oligocene early miocene and late miocene time comparison with the offshore tectonics indicates that the first two tectonic magmatic pulses coincide with periods of fast spreading at the pacific farallon boundary south of the shirley fracture zone we propose that increases in the spreading rate are related to periods of high subduction rate which in turn correspond to episodes of retreating subduction a retreating slab may have generated a flux of hotter asthenospheric material into the mantle wedge producing widespread melting at the base of the crust as well as intraarc extension in the overriding plate boundary conditions i e plate tectonics ultimately determined timing magnitude and orientation of extension whereas volcanic and tectonic styles are controlled by the internal structure of crustal blocks and by the gravitational and thermal effects of magmatism

interrelationship of sedimentary and volcanic deposits associated with tertiary extension in sonora mexico clastic sedimentary deposits and associated volcanic rocks record the progress of tertiary extension in the mexican state of sonora these deposits accumulated within basins located throughout the eastern two thirds of the state the rocks are slightly indurated conglomerates and sandstones clast types reflect the local highland exposures volcanic units that bound and are interlayered with the sedimentary rocks provide stratigraphic and time markers of basin evolution and by inference of marginal fault development this record has been examined in an east west belt across south central sonora the volcanic rocks occur in three distinct associations at the base and interbedded within the lower portions of the sedimentary sections are lava flows of dominantly basaltic andesite composition in most localities the overlying clastic sedimentary strata are conformable with these lava flows and we conclude that the magmatism was triggered by early faulting along the basin margins a second association present near the base of the sections comprises massive lava domes and flows of intermediate composition that contain distinctive phenocrysts of dark brown amphibole the domes apparently formed when viscous lava welled up along developing marginal basin faults in one case a transition was observed over a short distance from a structureless dome to a thick lava flow that is conformably interbedded with and sheds clasts laterally into the sedimentary section a third volcanic association comprises layers of rhyolitic lava flows and ignimbrites that overlie the coarsest and thickest lower portions of the sedimentary sections k ar ages for volcanic rocks in the two oldest of these settings indicate that each tertiary basin had a distinct history with no apparent regional geographic pattern in the rio yaqui basin where exhumation by the major river of sonora has exposed a full section of the volcanic and clastic sequences extension was possibly as old as 27 ma elsewhere deposition of sediments was underway in most basins by 24 ma and in all basins by 20 ma k ar ages of the younger rhyolitic volcanic rocks fall within a narrow range from 12 8 to 10 5 ma these ages provide a younger time limit to deposition of the coarser and thicker portions of the clastic sedimentary sections the mafic lava flows dominantly basaltic andesite are similar in major element composition to mafic lava flows that cap felsic sections within the sierra madre occidental volcanic field of western mexico and the mid tertiary datil mogollon volcanic field in southwestern new mexico they are higher in silica and lower in total alkalies than younger capping mafic lava flows within the gulf of california extensional province of western sonora and neogene basaltic lava flows in the datil mogollon field they are similarly distinct from basaltic dikes and lava flows that are clearly associated with basin and range faulting in trans pecos texas and in northern durango state a convergent margin magmatic arc was active in the present day gulf of california region between 24 and 11 5 ma the earliest extension in south central sonora was therefore occurring in a back arc setting continued sedimentation and rotation of volcanic strata indicate that extension continued throughout the region after 10 ma with little coeval volcanism

quaternary intra arc extension in the central trans mexican volcanic belt the trans mexican volcanic belt is an active volcanic arc related to subduction along the middle america trench and characterized by shallow seismicity and synvolcanic to postvolcanic extensional arc parallel faulting major intra arc basins within the central part of the belt between long 998w and 1028w are from west to east the cuitzeo and acambay grabens the aljibes half graben and the mezquital graben in this region 100 eastwest2striking 2 km long steeply dipping normal faults expressed by pronounced multi event fault scarps have been mapped of the 100 faults 65 displace rocks of known quaternary age younger than 1 6 ma and 22 cut rocks with a documented age younger than 750 ka known historical surface ruptures are limited to faults of the acambay graben overall the faults cause north south to north northwest2south southeast2oriented extension of 1 km which is distributed over a 30250 km wide zone that has 5210 faults in cross section and a relative extension of 3 quaternary vertical slip rates estimated for 13 of the faults have a mean of 0 07 mm yr they are highest at the southern margin of the cuitzeo graben the northern margin of the acambay graben and in the aljibes half graben where they measure between 0 16 and 0 18 mm yr the north south to north northwest2south southeast2oriented quaternary bulk extension rate of the system is likely to be 0 2 6 0 05 mm yr this fault system is in an initial stage of coalescence its western and eastern parts consist mostly of isolated fault segments whereas in the central part deformation is localized onto a few through going faults the longest of these are the venta de bravo fault 45 km and the acambay tixmadeje fault 34 km the short fault traces have a simple structure whereas the longer ones are commonly composed of two or more segments there is no obvious migratory pattern of quaternary fault activity which suggests that the entire region is tectonically active

area balance depth to detachment and strain in extension a new lost area cross section construction and validation technique is based on the area balance of a graben system formed above a detachment area balance requires that the area displaced below the original regional level of each bed in a graben the lost area be balanced by the displaced area at the boundary of the system given by the product of the displacement times the depth to detachment this relationship is a straight line on a plot of lost area versus depth the slope of the line is the displacement and the depth intercept is the depth of the detachment in general beds within the graben system undergo both visible and homogeneous layer parallel extension the homogeneous component can be determined from the width of the graben system the lost area and the depth to detachment conversely the detachment depth may be calculated from the total layer parallel extension and the lost area the techniques are illustrated by application to models and to very well constrained natural examples an earthquake induced slope failure in alaska and kilometric sized horsts and grabens in the black warrior foreland basin of alabama a half graben without footwall uplift and a full graben are the same on a lost area diagram a half graben with footwall uplift linked to the hanging wall extension requires that both the hanging wall and footwall areas be included to determine the detachment depth and displacement all the models show layer parallel extension e in the upper levels that are less than the crustal scale strain

some statistical properties of palaeomagnetic directions in icelandic lava flows summary this paper extends the efforts of kristjansson mcdougall to obtain a semi quantitative description of various long term 0 1 10 myr properties of the geomagnetic field in iceland due to the long time intervals between eruptions of the lavas recording the field local average and stochastic properties are emphasized rather than detailed aspects such as pole paths these properties can be represented by smooth functions of virtual pole latitude it is possible to describe both the frequency distribution of virtual poles in latitude and the estimated frequency of geomagnetic reversals by a simple process of random walk of the pole this is also in agreement with the observed long term symmetry of the reversal process and demonstrates that memory effects are unnecessary to account for the behaviour of the field the average transition time for the field isofthe order of 5 6000yr right handedness is a pervasive property of the field in iceland apparently increasing with age it is demonstrated that the size of palaeomagnetic collections in lava sequences has to be much larger than 20 units in order to yield consistent values of statistical parameters for mean palaeofield directions

causes of compression and extension behind trenches abstract three factors have hampered attempts to find the causes of compression and extension within overriding plates of subduction zones 1 1 adjacent subduction zones are often similar in many subduction parameters 2 2 proposed causal variables are themselves correlated and 3 3 physical models are limited by uncertainties in physical quantities and complexity of the subduction process nevertheless modern subduction zones provide a rich source of observations that help to distinguish among models we consider ten variables or combinations of variables which may control strain regime behind trenches convergence rate vc slab age as shallow slab dip ds absolute motion voa asthenospheric flow slab pull force as vc as voa vc ds voa and vc ds as ten observations are used to test these models a key prerequisite to these tests is the semiquantitative classification of the strain regime of each modern subduction zone into one of seven strain classes from active back arc spreading class 1 through active folding and foreland thrusting class 7 statistical analysis indicates that the modern variance in strain classes is not adequately accounted for by any single or dual variable model but the combination of vc ds and either asorvoa can account for 77 of this variance r 0 88 rapid changes in strain class both with time and along a subduction zone are similarly more consistent with the 3 variable models the role of vc is also supported by maximum compression orientations parallel to vc the role of ds is also supported by the tendency for oceanic overriding plates to be more extensional than continental overriding plates two factors suggest that the vc ds as model is more likely than vc ds voa 1 1 the dominance of one or more of the variables vc as andds in other subduction relations slab length earthquake moment trench relative depth deep dip and arc trench gap and 2 2 the correlation between strain class and maximum earthquake moment which is affected by vc and as however no confident discrimination between the two 3 variable models is yet possible

the tectonic expression slab pull at continental convergent boundaries examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style topographic elevation denudation metamorphism postcollisional convergence and foredeep basin geometry and facies the distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary at subduction boundaries where the rate of overall plate convergence is less than the rate of subduction termed here retreating subduction boundaries the transmission of horizontal compressive stress across the plate boundary is small and regional deformation of the overriding plate is by horizontal extension the tectonic expression of these retreating subduction boundaries includes topographically low mountains little erosion or denudation low grade to no metamorphism little to no involvement of crystalline basement in shortening little to no postcollisional convergence anomalously deep foredeep basins and a protracted history of flysch deposition within the adjacent foredeep basin analysis of deflection and gravity data across three retreating subduction boundaries apennine carpathian and hellenic systems shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km carpathians 50 and 150 km apennines and 50 and 250 km hellenides the total mass anomalies represented by the slabs are approximately 3 1012 6 1012 and 12 1012 n m respectively the slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland and partially supported by dynamic viscous stresses in the asthenosphere at subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction termed here advancing subduction boundaries the transmission of horizontal compressive stress across the plate boundary is large and regional deformation of the overriding plate is by horizontal shortening the tectonic expression of these advancing subduction boundaries includes topographically high mountains antithetic thrust belts large amounts of erosion and denudation exposure of high grade metamorphic rocks at the surface extensive deformation of crystalline basement to midcrustal depths protracted postcollisional convergence tens of millions of years and a protracted history of molasse deposition within the adjacent foredeep basins analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere western to eastern and southern alps and himalayas shows that the thrust sheets have been translated for great distances over the foreland lithosphere relative to the point at which the subduction forces are applied thus obscuring any flexural and gravity signals from the subducted slab however it appears that far field stresses presumably related to global plate motions drive most of the convergent motion across these subduction boundaries the concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved appendix b is available with entire article on microfiche order from the american geophysical union 2000 floridaavenue n w washington d c 20009 document t92 004 2 50 payment must accompany order

stratigraphy and geochronology of the comondu group near loreto baja california sur mexico abstract upper oligocene to middle miocene volcanic and sedimentary rocks in the loreto region baja california sur are widely exposed and make up the comondu group as redefined here following mcfall the comondu group is part of a volcanic arc and forearc basin that formed along the northwestern margin of mexico regional to detailed scale mapping stratigraphic analysis and geochronology in a 10 20 km wide and 70 km long belt from the gulf escarpment to the coast near loreto reveal three main units in the comondu group and a composite thickness of 1 5 2 km 1 the lower clastic unit 30 19 ma contains 200 300 m of fluvial sandstone and conglomerate with probable local eolian deposits and numerous felsic tuffs and basalt flows that accumulated in a forearc basin 2 the middle breccia and lava flow unit 19 15 ma is up to 750 m thick and consists of massive andesite breccia that was deposited as proximal debris flows interbedded with minor andesite lava flows 3 the upper lava flow and breccia unit 15 12 ma consists almost entirely of up to 600 meters of andesite lava flows a few kilometers west of loreto that are part of a composite volcano north and south of loreto for up to 25 km the unit is composed of andesite lava flows and massive andesite breccia that formed from thick debris flows near that and other volcanic centers the three units of the comondu group in the loreto area correlated with the salto pelones and ricason formations of the bahia concepcion area the lower clastic unit of this study is time equivalent to the marine san gregorio el cien and isidro formations on the western and southern baja california peninsula the middle and upper units of the loreto area are the proximal equivalent to the type section of the comondu group near the village of comondu during deposition of the lower part of the comondu group the arc lay to the east on mainland mexico in the sierra madre occidental and then migrated to the position of the gulf of california at about 25 ma at 19 ma the proximal part of the arc migrated abruptly 50 km westward to approximately the position of carmen island and at 15 ma the arc once again stepped west forming eruptive centers along the coast near loreto until 12 ma also at 15 ma the proximal forearc region changed from aggradational to neutral or erosional such that no rocks of the upper unit are preserved in the gulf escarpment

late miocene volcanism and intra arc tectonics during the early development of the trans mexican volcanic belt abstract the early stage of the trans mexican volcanic belt hereafter tmvb is marked by widespread mafic to intermediate volcanism emplaced between 11 and 7 ma from the pacific coast to the longitude of mexico city to the north of the modern volcanic arc petrological and geochronological data support the hypothesis that this volcanism made up a unique late miocenic central mexican comagmatic province mafic lavas at the mouth of the gulf of california and along the northwestern sector of the tmvb made up the nayarit district which includes calc alkaline to transitional varieties the central sector of the tmvb is characterized by two basaltic districts the jalisco guanajuato and the queretaro hidalgo which are distinguished from the westernmost ones by their lower nb la and generally lower hfse lile values as well as by spider diagrams characterized by larger negative spikes at th ta nb and ti the surface occurrence of the late miocene basalts appears to be controlled by pre existing zones of crustal weakness that channeled the mafic magmas field observations suggest that these structures have been reactivated in a transtensional fashion induced by differential tectonic motion of crustal blocks to the south and to the north of the tmvb starting from 12 ma the tmvb separates a northern tectonic domain subject to the developing divergent pacific north america plate boundary from a southern tectonic domain characterized by oblique subduction of the rivera and cocos plates apparently far field stresses related to these complex plate boundaries reactivated older suture zones allowing rapid uprise of mantle derived magmas the subduction related signature shown by miocene mafic lavas of the jalisco guanajuato district argues against the existence of mantle plumes beneath this sector of the north america plate on the other hand the occurrence in the western tmvb and in the guadalajara region of a large volume of mafic magmas which sometimes show characteristics transitional to ocean island basalts could be due to passive upwelling of the subslab asthenosphere that might have interacted with subduction related magmas and continental lithosphere to produce the observed basaltic varieties subslab magmas may have flowed through slab free areas along the northern and eastern edges of the subducting rivera plate which in the late miocene was already detached from the farallon plate remnants and diverging from the cocos plate

large wave forms from the fish canyon tuff colorado wave like bedforms of large amplitude and wavelength 6 m 60 m occur in an exposure at the base of the fish canyon tuff colorado an ignimbrite ash flow tuff of great volume 3 000 km 3 the bedforms are located some 30 40 km from the center of the source caldera they are interpreted to be deposits from a series of large pyroclastic surges precursors to the main pyroclastic flows the pyroclastic surge beds have bigger wave forms than those previously recognized such megawaves were perhaps produced by violent explosions related to major unroofing of a magma chamber the depositional sequence of this great ignimbrite forming event is similar to that of much smaller eruptions and implies a similar eruptive sequence

k ar age studies in the fresnillo silver district zacatecas the fresnillo district one of the major silver producers in the world contains a spectrum of hydrothermal mineralization ranging from skarn and pb zn replacement orebodies to epithermal veins and paleohot spring deposits the purpose of the k ar age study was to elucidate the temporal relationship between the magmatic and hydrothermal events the oldest lithologic units in the area are mesozoic marine sediments and mafic volcanics cut by small intrusions of granodioritic monzonitic composition and unconformably overlain by mid tertiary felsic volcanics adjacent to the latter are some of the more important orebodies of the fresnillo district the k ar measurements demonstrate an evolving pattern of initial felsic volcanism 38 4 or 0 8 m y followed by igneous intrusion 32 33 m y hydrothermal activity 29 31 m y and finally postmineralization felsic volcanism 27 28 m y these data are in agreement with the sequence of events that can be deduced from the local geologic relationships the k ar ages also emphasize the fundamental association between intrusive magmatism and metallization in the fresnillo district

rheomorphism of welded tuffs abstract peralkaline welded tuffs from the islands of gran canaria canary islands and pantelleria italy show abundant evidence for post depositional flow it is demonstrated that rheomorphism or secondary mass flowage can occur in welded tuffs of ignimbrite and air fall origin the presence of a linear fabric is taken as the diagnostic criterion for the recognition of the process deposition on a slope is an essential condition for the development of rheomorphism after compaction and welding internal structures produced during rheomorphic flow can be studied by the methods of structural geology and show similar dispositions to comparable features in sedimentary slump sheets it is shown that secondary flowage can occur in welded tuffs emplaced on gentle slopes provided that the apparent viscosity of the magma is sufficiently low compositional factors favor the development of rheomorphism in densely welded tuffs of peralkaline type

magnetic anisotropy in rhyolitic ignimbrite snake river plain implications for using remanent magnetism of volcanic rocks for correlation paleomagnetic studies and geological reconstructions some of the field and laboratory expenses for this work were covered by the natural environment research council nerc grant ne g005372 1 the salary and school fees for the first author were covered in part by a national science foundation nsf grant ear 443549 81179 awarded to x zhao

igneous sulfides in the fish canyon tuff and the role of sulfur in calc alkaline magmas pyrrhotite is a common inclusion in several phenocrystal phases from the fish canyon tuff a voluminous homogeneous ash flow erupted from the central san juan field the fugacities of various gaseous species have been calculated using the composition of the pyrrhotite iron titanium oxides and biotite sulfur and oxygen fugacities fall very near the sulfur condensation curve implying that liquid sulfur may have been present in the parent magma high sulfur fugacities during melt formation can cause high oxygen fugacity in calc alkaline magmas due to the breakdown of pyrite at high temperatures during cooling of such magmas separation of so 2 rich gases causes a reduction in oxygen fugacity significant amounts of sulfur are available during degassing and represent a major source of sulfur for related ore deposits the ultimate source of the high sulfur and oxygen activities is probably pyrite and magnetite deposited during the waning stages of hydrothermal circulation at active oceanic ridges other ash flows and calc alkalic volcanic rocks should be carefully examined for similar sulfide phases

miocene shearing along the northern boundary of the jalisco block and the opening of the southern gulf of california new geologic and structural mapping in western mexico shows that the tectonic boundary between the eocene early miocene sierra madre occidental volcanic province and the cretaceous paleocene jalisco block was reactivated as a transfer zone accommodating a limited amount of lateral motion in middle and late miocene time transpressional folding left lateral strike slip faulting and minor thrusting occurred between 14 5 and 11 5 ma during initial rifting at the mouth of the gulf of california inferred right lateral transtension between 12 and 9 ma took place concurrently with widespread alkaline volcanism and east west extension at the gulf mouth and east of the jalisco block these events can be explained in light of the long extensional history that led to the separation of baja california from mainland mexico since the birth of the magdalena microplate about 14 5 ma left lateral shearing at the sierra madre occidental jalisco block boundary indicates that initial 14 12 ma rifting at the mouth of the gulf was accomplished mainly by east southeast motion of the block this was caused by differential stresses applied to the north america plate at the two sides of the subducting magdalena cocos plate boundary as a consequence of the waning of subduction of the magdalena plate subsequent right lateral transtension at the sierra madre occidental jalisco block boundary and extension to the east imply a limited west northwest motion of the block which possibly accounts for the discrepancy between slow rifting at the mouth of the gulf and fast northwest motion of baja california

distinguishing and correlating deposits from large ignimbrite eruptions using paleomagnetism the cougar point tuffs mid miocene southern snake river plain idaho usa in this paper we present paleomagnetic geochemical mineralogical and geochronologic evidence for correlation of the mid miocene cougar point tuff cpt in southwest snake river plain srp of idaho the new stratigraphy presented here significantly reduces the frequency and increases the scale of known srp ignimbrite eruptions the cpt section exposed at the black rock escarpment along the bruneau river has been correlated eastward to the brown s bench escarpment six common eruption units and cassia mountains three common eruption units regions of southern idaho the cpt records an unusual pattern of geomagnetic field directions that provides the basis for robust stratigraphic correlations paleomagnetic characterization of eruption units based on geomagnetic field variation has a resolution on the order of a few centuries providing a strong test of whether two deposits could have been emplaced from the same eruption or from temporally separate events to obtain reliable paleomagnetic directions the anisotropy of anhysteretic remanence was measured to correct for magnetic anisotropy and an efficient new method was used to remove gyroremanence acquired during alternating field demagnetization

determination of the tectonic stress tensor from slip along faults that obey the coulomb yield condition a new method to calculate the stress tensor associated with slip along a population of faults is derived here the method incorporates two constraints first the stresses in the slip direction satisfy the coulomb yield criterion and second the slip occurs in the direction of maximum shear stress along the fault the computations provide the complete stress tensor normalized by the vertical stress and evaluation of the mean coefficient of friction and the mean cohesion of the faults during the time of faulting the present method is applied to three field cases dixie valley nevada wadi neqarot israel and yuli microearthquakes taiwan it is shown that the coefficieet of friction of the three cases varies between 0 1 to 0 8 further it is demonstrated that previous stress inversion methods implicitly assume zero friction along the faults

anomalous magnetic properties in rocks containing the mineral siderite paleomagnetic implications sampling and magnetic measurement of a 1 m bed in a new road cut of the upper cretaceous austin chalk northeastern texas has yielded anomalous magnetic results initial measurement of the anisotropy of magnetic susceptibility ams indicated unusually high anisotropies and low bulk susceptibilities characteristic of a magnetocrystalline anisotropy which might be expected for siderite feco3 natural remanent moments nrm for these samples were low 1 10 4 a m2 and directions were typical for samples which had acquired a normal geomagnetic field overprint at the site periodic remeasurement of the nrm yielded an increase in moment for some samples and periodic reversals of rm in a direction parallel or antiparallel to the ambient field in the laboratory isothermal remanent moments of these samples saturates at low induction values 200 mt ams remeasurement over a period of weeks to months revealed a general decrease in the anisotropy magnitudes an increase in susceptibility and a change in principal axis orientations these data were compared with heating oxidation experiments on austin chalk siderite ore and clastic sediments with siderite cements all of these results are consistent with the x ray diffraction and mossbauer data which indicate the presence of siderite in the austin chalk samples the mossbauer spectra of the samples obtained at room temperature and 78 k indicate the presence of approximately 80 pyrite 10 siderite and 10 of a clay component possibly chlorite when only the iron bearing mineral components are considered after sampling exposure to the air and subsequent oxidation in the laboratory the siderite in austin chalk samples appears to have altered to fe2o3 maghemite or fe3o4 magnetite increasing the magnetic moment and changing the nrm and ams directions in the samples the continuing changes appear to reflect a conversion from the less stable fe2o3 to fe2o3 hematite or oxidation of fe3o4 to fe2o3 we infer from our study that oxidation of siderite bearing limestone e g during the formation of unconformities or due to low magnitude tectonic events may produce a very stable secondary magnetic moment in the rock such changes may be very important when interpreting paleomagnetic data from limestones

extensional segmentation of the main gulf escarpment mexico and united states periodic reversal of upper plate transport direction on miocene and younger extensional fault systems occurs in the northwestern gulf extensional province baja california and adjacent united states with segments in which the peninsular ranges are in either footwall or hanging wall positions accommodation zones separate these segments and are defined by changes of structural style and topography in the peninsular ranges and or the gulf extensional province these and other characteristics allow speculative definition of segments 50 150 km long along the baja california peninsula

synextensional pliocene pleistocene eruptive activity in the camargo volcanic field chihuahua mexico the camargo volcanic field is the largest mafic alkalic volcanic field in the mexican basin and range province and the relationship between volcanism and normal faulting is especially strong the camargo volcanic field lies in the northern part of the province midway between the sierra madre occidental and trans pecos texas it is formed by pliocene pleistocene 4 7 0 09 ma intraplate mafic alkalic volcanic rocks some of which contain peridotite pyroxenite and granulite xenoliths the volcanic field covers 3000 km2 and has an estimated volume of 120 km3 erupted from 300 recognized vents twenty six new 40ar 39ar age determinations for the camargo volcanic field and its environs show that volcanic activity began in the southwest part of the field and shifted toward the northeast at 15 mm yr the average magmatic eruption rate during growth of the field was 0 026 km3 k y the camargo volcanic field lies within an accommodation zone with west dipping faults and east tilted blocks to the north and east dipping faults and west tilted blocks to the south these faults are expressed in the volcanic field by a n30 w trending graben with scarps up to 100 m high through its central part volcanism and faulting were at least in part coeval and younger volcanic products commonly drape fault scarps that cut earlier lavas normal faulting is bracketed between 4 7 and 2 1 ma and may have also migrated northeastward estimated vertical slip rates on four pliocene faults range from 0 03 mm yr a likely long term rate to 1 67 mm yr interpreted as a short term rate operative during periods of active faulting northwest striking normal faults that cut alluvial fan deposits and pleistocene lavas in the northern camargo volcanic field and geomorphic evidence for recent uplift to the south of the volcanic field suggest that the region is still extending

paleomagnetic directional dispersion produced by plastic deformation in a thick miocene welded tuff southern nevada implications for welding temperatures two voluminous rhyolitic to quartz latitic ash flow sheets the tiva canyon and topopah spring members of the paintbrush tuff were erupted from the miocene claim canyon cauldron in southern nevada although lithologically similar these units differ greatly in their recording of the ancient geomagnetic field the reversely magnetized tiva canyon member yields remanent directions that are well grouped both within and between sites the normally magnetized topopah spring member however yields remanent directions that vary both laterally and vertically remanent directions from a 100 m thick section of nonwelded to partially welded topopah spring are well grouped precision parameter k 200 and give a mean direction d 293 i 69 that is probably a good approximation of the geomagnetic field at the time of emplacement and cooling in three 300 m thick sections of more densely welded tuff inclinations from the upper parts of the topopah spring are steep 70 but flatten with depth to about 20 and then steepen abruptly near the base of the unit in general the directional dispersion increases with the thickness of the section k 30 for sections that are about 300 m thick much of the directional variation in the topopah spring can be attributed to an inclination error produced by subblocking temperature rotation of the magnetic carriers during welding the possibility of subblocking temperature deformation in an ash flow sheet is enhanced by 1 high blocking temperature spectra of the primary magnetic phases 2 relatively low emplacement temperatures and 3 great stratigraphic thickness thermal demagnetization results from the topopah spring member indicate that welding may have continued to temperatures as low as 475 c in the basal vitrophyre of the ash flow sheet remanent directions from the tiva canyon member were unaffected by the welding process because this unit possesses significantly lower blocking temperatures and does not attain the great thicknesses of the topopah spring

crystallization remanent magnetization during the transformation of maghemite to hematite we have studied crystallization remanent magnetization crm in the fe2o3 fe2o3 transformation starting from synthetic equidimensional maghemite crystals of single domain size median diameter of 245 a inversion of maghemite to hematite on heating was indicated by an exothermic peak in differential thermal analysis dta at 555 c in crm experiments samples of dispersed maghemite were heated in zero field to temperatures between 300 c and 604 c held for 3 hours in a 50 t field hcrm and cooled in zero field to 20 c the intensity of the resulting crm mcrm increased with increasing conversion to hematite reaching a maximum of 784 a m 1 0 784 emu cm 3 after the 556 c run then decreased to 1 3 a m 1 0 0013 emu cm 3 after the 604 c run maximum crm intensity and the most rapid inversion indicated by dta both occurred around 555 c mcrm partially inverted samples was always parallel to hcrm hematite cannot explain the high intensity and moderate coercivity of the crm the bulk of the crm seems to be carried by maghemite and to be acquired viscously rather than by grain growth in a second set of experiments crm was produced in the presence of a preexisting anhysteretic remanent magnetization arm marm perpendicular to hcrm the resulting remanence mt was a composite of surviving anhysteretic remanent magnetization arm plus crm directed parallel to hcrm rather than a single vector of intermediate direction mt can be synthesized by orthogonally combining crms measured in the first set of experiments with arms heated in zero field phase coupling of maghemite and hematite is not necessary to explain the main features of crm with or without an m initial remanence

motion of the rivera plate since 10 ma relative to the pacific and north american plates and the mantle abstract to better understand the influence of rivera plate kinematics on the geodynamic evolution of western mexico we use more than 1400 crossings of seafloor spreading magnetic lineations along the pacific rivera rise and northern mathematician ridge to solve for rotations of the rivera plate relative to the underlying mantle and the pacific and north american plates at 14 times since 9 9 ma our comparison of magnetic anomaly crossings from the undeformed pacific plate to their counterparts on the rivera plate indicates that significant areas of the rivera plate have deformed since 9 9 ma dextral shear along the southern edge of the plate from 3 3 2 2 ma during a regional plate boundary reorganization deformed the rivera plate farther into its interior than previously recognized in addition seafloor located north of two rupture zones within the rivera plate sutured to north america after 1 5 ma anomaly crossings from these two deformed regions thus cannot be used to reconstruct motion of the rivera plate finite rotations that best reconstruct pacific plate anomaly crossings onto their undeformed counterparts on the rivera plate yield stage spreading rates that decrease gradually by 10 between 10 and 3 6 ma decrease rapidly by 20 after 3 6 ma and recover after 1 ma the slowdown in pacific rivera seafloor spreading at 3 6 ma coincided with the onset of dextral shear across the then incipient southern boundary of the rivera plate with the pacific plate the available evidence indicates that the rivera plate has been an independent microplate since at least 10 ma contrary to published assertions that it fragmented from the cocos plate at 5 ma motion of the rivera plate relative to north america has changed significantly since 10 ma in concert with significant changes in pacific rivera motion a significant and robust feature of rivera north america motion not previously recognized is the cessation of margin normal convergence and thus subduction from 2 6 to 1 0 ma along the entire plate boundary followed by a resumption of trench normal subduction along the southern half of the rivera north america plate boundary after 1 0 ma motion of the rivera plate relative to the underlying mantle since 10 ma has oscillated between periods of landward motion and seaward motion the evidence suggests that the torque exerted by slab pull on this young and hot oceanic plate is either minimal or is effectively counterbalanced by forces that resist its motion

the sanfandila earthquake sequence of 1998 queretaro mexico activation of an undocumented fault in the northern edge of central trans mexican volcanic belt abstract a sequence of small earthquakes occurred in central mexico at the northern edge of the trans mexican volcanic belt tmvb in the state of queretaro during the first 3 months of 1998 medium to large events in the continental regime of central mexico are not common but the seismic history of the region demonstrates that faults there are capable of generating destructive events the sequence was analyzed using data from a temporary network with the goals of identifying the causative fault and its relation to regional tectonics employing a waveform inversion scheme adapted from a method used for regional studies we found that the source mechanisms conform to the style of faulting i e extension in the e w direction representative of the taxco san miguel allende fault system this system has been proposed as the southernmost extension of the basin and range br province the spatial distribution of hypocenters and source mechanisms indicate that the seismogenic segment was a fault with an azimuth of approximately 334 with almost pure dip slip since events which occurred just south from this region show features which are consistent with tmvb tectonics i e extension in an n s direction the sequence may mark the boundary between the tmvb and br stress domains

geology tectonic environment structural controls in the san martin de bolanos district jalisco the san martin de bolanos mining district is located in the southernmost portion of the barranca section of the sierra madre occidental province and is approximately 190 km by road from guadalajara jalisco mexico the volcanic rocks consisting of intercalated ash and lava flows are early miocene in age and mineralization shows a close affinity to those of andesitic composition other lithologies present include extensive postmineralization capping ash flows and a bimodal suite of basalts and rhyolites which appear to be restricted to the floor of a structural depression these basalts are subalkaline to weakly alkaline in composition but a rhyolitic volcanic neck of late miocene to pliocene age is strongly alkaline situated within the valley is a large accumulation of stratified volcaniclastics and conglomerates along the western escarpment are laharic flows which unconformably overlie the stratified sediments structurally the district is situated in a large north northeasterly trending graben that formed in miocene time with later readjustments of pliocene age this graben is a downdropped block 35 to 40 km in width and 150 km in length the five stages of graben evolution consist of i oligocene miocene ignimbrite flare up ii arching fracturing and fissure vein mineralization iii postmineralization ash flow tuff emplacement iv partial subsidence of the arched structure due to volcano tectonic collapse and partial overlap with later collapse due to extension that began in middle to late miocene time and v continued minor periodic subsidence during the final stages of valley development apparent back arc extension or incipient spreading ridge jumps following convergent plate motion are most likely responsible for the structural grain ore mineralization consists primarily of stromeyerite argentitc silver oxides and small amounts of gold in an epithermal environment quartz is the primary gangue mineral and there are lesser amounts of specularitc sphalerite fluorite chalcopyrite galena pyrite and barite electron microprobe analyses show that the average value of silver in galena is only 0 08 wt percent ag and the iron content of sphalerite is 0 95 wt percent fe the veins are steeply dipping and the fractures pinch and swell localizing the metals in the dilations the late mineralization fluids at san martin de bolafios were evidently of low temperature 140 degrees c and salinity 0 5 equiv wt nacl as interpreted from the fluid inclusion study of fluorite although the source of the mineralizing fluids is unknown an adjacent rhyolitic intrusion emplaced in the western escarpment of the graben may be genetically related to mineralization the total vertical extent of mineralization in the area is not known but near vertical mineralized intervals are known to exceed 250 m dating of pre and postmineralization volcanic flows indicates that the mineralization occurred within the time span of 23 7 to 21 3 m y it does not appear from a comparison of the attitudes of the fault and fracture patterns of the graben structure to those containing mineralization that the mineralization was genetically related to the development of the graben structures since metallization predates the main taphrogenic episode

estimates of flow direction for calc alkaline welded tuffs and paleomagnetic data reliability from anisotropy of magnetic susceptibility measurements central san juan mountains southwest colorado abstract flow directions are estimated from the measurement of the magnetic fabric of 106 samples collected at 18 sites in four welded tuff units in the central san juan mountains of southern colorado the estimates assume that the tuffs generally flowed directly away from the extrusive vents and that the lineations of magnetic grains within the tuffs represent the flow direction at individual sites errors in the estimation may arise from topographic variation rheomorphism post emplacement mass flow within the tuff and other factors magnetic lineation is defined as the site mean anisotropy of magnetic susceptibility maximum azimuth a test on the flow directions for individual units is based on the projection of lineation azimuths and their intersection within or near the known source caldera for the tuff this test is positive for the four units examined paleomagnetic results for these tuffs are probably reliable indicators of the geomagnetic field direction in southwest colorado during the time 28 2 26 5 ma of emplacement

counterclockwise rotations in the late eocene oligocene volcanic fields of san luis potosi and sierra de guanajuato eastern mesa central mexico abstract we used paleomagnetic and structural data to investigate the late eocene oligocene tectonic evolution of the mesa central area in mexico the mesa central was affected by nw trending faults tepehuanes san luis fault system coeval with a late eocene oligocene ignimbrite flare up and by post 27 ma nne trending grabens related to the basin and range we obtained reliable paleomagnetic directions from 61 sites within the late eocene oligocene volcanic series 30 to 27 ma of the san luis potosi volcanic field and sierra de guanajuato for each site we also measured the anisotropy of magnetic susceptibility ams tilt corrections were made using ams data for 33 sites where in situ bedding measurements were not available paleomagnetic directions indicate counterclockwise rotations of about 10 with respect to stable north america after 30 25 ma structural data suggest that the volcanic succession was mainly affected by normal faults however we also found evidences for oblique or horizontal striae showing a left lateral component along nw trending faults and a right lateral component along ne trending faults both motions are consistent with a n s extension oblique to the tepehuanes san luis fault system previous paleomagnetic studies in northern and southern mexico show the prevalence of minor left lateral shear components along regional scale transpressional and transtensional lineaments our paleomagnetic data may reflect thus small vertical axis rotations related to a minor shear component coeval with the oligocene intra arc extension in central mexico

jurassic to holocene tectonics magmatism and metallogeny of northwestern mexico the present metallic distribution in northwestern mexico is the culmination of superposed magmatism tectonism erosion and burial over more than 150 m y detailed palinspastic reconstructions of preextensional configurations the first study of its kind for this region clarify the interplay among these features on the present distribution and character of mineralized geologic systems this new synthesis goes beyond previous metallogenic investigations of northwestern mexico by separating events into specific timing and structural relationships and by restoring the geology to its preextensional configuration metallogenic factors such as enrichment preservation and erosion play major roles in the present distributions and for the first time are related to the overall metallogenic framework of northwestern mexico the analysis concludes that modern metallogenic patterns are the result of the complex superposition and subsequent redistribution of geologic systems in a way that is related directly to the regional history rather than simply metallic belts or interpreted angle of a subducting slab three main extensional events in the oligocene holocene have been restored and the palinspastic distributions have been analyzed reconstructions reveal the following 1 mineralization events igneous centers and sedimentary sequences are continuous across the gulf of california and other areas with large amounts of extension 2 middle tertiary gold silver mineralization in baja california may be the western part of the sierra madre oc present address bhp billiton avenida america vespucio sur 100 8th floor las condes santiago chile e mail john mark g staude bhpbilliton com cidental metallogenic province thus expanding the previously recognized extent of this province s mineralization 3 late cretaceous early tertiary porphyry copper deposits and intrusive centers form a narrower belt than previously noted and are traceable for over 400 km with parts of the belt buried beneath the younger sierra madre occidental volcanic fields 4 interpreted alignments of older geologic features including lineaments of ore deposits are displaced in the reconstructions 5 sedimentary rock hosted gold deposits and low angle detachment gold systems are closely related and occur around core complexes by using structurally restored time slices it becomes clear that older deposit types tend to be those formed at greater depths and more proximal to intrusions whereas younger deposits formed at shallower depths are less eroded and are more commonly volcanic rock hosted these characteristics express themselves in the regional distribution of deposit types second mineralization of widely differing ages is spatially superposed commonly associated with coeval magmatic and tectonic events the structural and magmatic events together with paleodistribution of ore deposits define a new framework to interpret the metallogenic history of northwestern mexico

large magnitude oligo miocene extension in southern sonora implications for the tectonic evolution of northwest mexico new structural and 40ar 39ar data from the santa rosa area southeastern sonora shed light on the magnitude and timing of extensional tectonism episodic magmatic activity from the late cretaceous to the miocene is recorded by a 3 4 km thick structural stratigraphic section that includes a composite laramide granodiorite batholith and its andesitic wall rocks early to middle tertiary rhyolite ignimbrites and silicic domes and late oligocene to middle miocene mafic lavas interbedded with tuffaceous sediment and fanglomerate the older units are complexly faulted and steeply tilted by several generations of normal faults palinspastic reconstructions indicate cumulative extension of 90 early gently dipping nw striking normal faults document major extension oriented n50 e angular unconformities and growth fault relations within oligo miocene sequences bracket this extension between 26 and 20 ma younger widely spaced ns to nnw trending high angle normal faults cut the previously faulted and tilted sections and produced modest 10 15 e w extension that is largely bracketed between 20 and 17 ma the magnitude of neogene extension in this region is greater and the timing is older than previously recognized reconnaissance of other areas between the sierra madre occidental and the coast at this latitude suggests that most extension in sonora occurred between 27 and 12 ma while remnants of the farallon plate were still being subducted plate tectonic models that predict substantial extension in sonora during proto gulf 10 5 ma transtensional deformation may need to be reevaluated an alternative model presented here is that baja california began moving with pacific plate motion shortly after the 11 ma termination of subduction and that northwest motion of baja relative to mainland mexico might total 500 km distributed across the previously extended oligo miocene magmatic arc

pre pliocene extension around the gulf of california and the transfer of baja california to the pacific plate late miocene 12 5 ma extension around the edges of the gulf of california has been alternatively attributed to basin and range extension back arc extension or development of the pacific north america plate boundary this extension was ene directed and similar in structural style to extension in the basin and range province timing constraints permit nearly synchronous onset of this deformation in a belt extending sse from northernmost baja california to the mouth of the gulf where this extensional faulting continued through pliocene time to the present synchronous with motion on the modern transform plate boundary in the gulf of california no change in direction of extension can be resolved revised constraints on pacific north america plate motion support the development of this late miocene extension as a component of pacific north america displacement that could not be accommodated by strike slip displacement along the existing plate boundary west of the baja california peninsula this scenario implies that transfer of baja california from the north america plate to the pacific plate was a gradual process beginning about 12 10 ma when motion of the pacific plate relative to north america was partitioned into separate regimes of strike slip and dip slip displacement on opposite sides of baja california

palaeomagnetism and magnetic properties of the cappadocian ignimbrite succession central turkey and neogene tectonics of the anatolian collage abstract the cappadocian ignimbrite succession of central southern anatolia comprises at least nine major and two minor calc alkaline rhyolitic sheets emplaced at 1 2 ma intervals between 11 2 and 1 1 ma it records the last phase of neotethyan subduction during final emplacement of the tauride orogen in southern turkey this study reports magnetostratigraphy and describes associated rock magnetic properties remanence resides in ti poor titanomagnetites haematisation is locally produced by post emplacement oxidation but does not contribute significantly to the palaeomagnetic signature although secondary processes within the ignimbrite sheets have produced composite isothermal remanent magnetisation spectra and variable intensities of magnetisation weak anisotropy of magnetic susceptibility describes tensors with maximum axes close to bedding and minimum axes perpendicular to this plane directions of kmax with weak imbrication mostly suggest flow away from centres of eruption located by gravity and remote sensing older ignimbrites upper and lower goreme akdag zelve from the cardak centre are all of normal polarity later ignimbrites partly erupted from the derinkuyu centre comprise the sarimaden r cemilkoy r tahar r kizilkaya r incesu n and valibaba sofular r ignimbrites the overall reversed group mean is d i 174 51 n 10 units r 9 84 95 6 6 k 55 and all magnetisation directions from the upper goreme 9 0 ma onwards are rotated anticlockwise with respect to eurasian and african palaeofields this sense of rotation characterises most of central anatolia and averages 9 5 in this sector the rotation rate from 8 to 1 ma bp was 1 25 ma but it appears to have accelerated during the latter part of the quaternary to about an order higher than rates determined from gps rotation has resulted from extrusion of fault blocks during tectonic escape of the anatolian collage to the southwest and followed crustal thickening as the afro arabia plate has continued to impinge differentially into eurasia cenozoic magnetic inclinations are systematically shallower across anatolia than inclinations expected from the geocentric dipole model and the apparent polar wander of bordering major plates only part of this difference 400 km can be accommodated by northward movement and crustal thickening in central anatolia since mid miocene times shallow inclinations observed in the aegean region extend into turkey they are observed in young volcanics and appear to reflect a regional geomagnetic anomaly the pattern of neotectonic declinations across anatolia shows rotations that are strongly anticlockwise rotated in the east near the arabian pincer but diminish towards the west to become zero or slightly clockwise at the western extremity of the collage rotations also seem to become generally younger towards the south crustal deformation has therefore been distributed and the net effect of terrane extrusion to the west and south has been to expand the curvature of the tauride arc and by inference the cyprus arc

palaeomagnetism of the upper volcanic supergroup southern part of the sierra madre occidental mexico ash flow tuffs or ignimbrites have been recently proposed to be a good material for palaeointensity determination in this paper we present a multidisciplinary study combining geochronology petrology rock magnetism and palaeomagnetism carried out on oligocene to early miocene ignimbrites and related flows from the southern part of the sierra madre occidental two new 40ar 39ar ages were determined for ignimbrites 20 4 0 2 ma the youngest age obtained so far in this area and 29 2 0 5 ma density measurements as a proxy for welding proved to be extremely useful to estimate the emplacement temperature and the origin of the magnetizations carried by the ignimbrites after alternating field and thermal demagnetizations the mean palaeomagnetic pole lat 66 8 n long 180 5 e kappa 142 a95 6 3 calculated for the period 28 31 ma is in close agreement with our only miocene determination comparison with the north america synthetic apparent polar path indicates a net counter clockwise vertical axis rotation of about 10 4 compared to stable north america which occurred likely during the last extensional episode in the late miocene ca 12 9 ma palaeointensity estimates obtained with the thellier coe method are mainly questionable and should not be used for global interpretation therefore these ignimbrites are not a viable material for reliable palaeointensity determinations

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

origin of the joya honda maar san luis potosi mexico abstract joya honda is a quaternary maar of unusual type from the mexican basin and range province its 300 m deep crater is excavated in cretaceous limestones the surrounding tephra deposit which in places is 100 m thick begins with a series of weakly indurated pyroclastic surge and fall layers that we interpret as dry surge deposits these are overlain by the main sequence of strongly indurated massive tuff breccias that we interpret as wet surge deposits joya honda formed subaerially from the interaction of groundwater with rapidly ascending intraplate type basanitic magma carrying peridotitic mantle xenoliths local aquifer characteristics controlled the style of eruption and the nature of the deposits groundwater in the limestone hosted aquifer beneath joya honda was apparently contained within solution enhanced fractures at the onset of the eruption magma began to interact with a moderate amount of groundwater producing the dry surge deposits which are typical of deposits found at many maars and tuff rings as the eruption continued the crater grew and the hydromagmatic blasts fractured the limestones around the explosion foci a marked increase in the water magma ratio of the system followed when a large fracture or a portion of the limestone with enhanced secondary permeability was intersected by the expanding crater subsequent phreatomagmatic explosions occurred in a system with groundwater flow rates several orders of magnitude larger than in the initial dry surge stage at the maar rim these wet eruptions led to the emplacement of massive tuff breccias through a combination of fallout surges and mudflows these steeply dipping tuff breccias are similar to deposits found at many tuff cones juvenile clasts in the near vent deposits show marked upward increases in both hydration palagonitization and vesicularity the increased palagonitization with height in the section appears to be a consequence of the overall increased wetness of the eruption with time correlating with greater carbonate cementation and lithification in the upper part of the deposit the transition toward higher vesicularity is interpreted as evidence of a gradual reduction in the confining pressure for the ascending magma prior to explosive fragmentation perhaps related to unroofing during progressive excavation of the overlying maar crater thus joya honda does not support maar formation models that invoke downward displacement of explosion foci caused by formation of a cone of depression in the aquifer in order to maintain the confining pressure for the hydromagmatic blasts