stable organic carbon isotope stratigraphy across oceanic anoxic event 2 of demerara rise western tropical atlantic ocean drilling program odp leg 207 recovered expanded sections of organic carbon rich laminated shales on demerara rise western tropical atlantic high resolution organic carbon isotope and total organic carbon toc records are presented which span the cenomanian turonian boundary interval ctbi including the oceanic anoxic event oae 2 from four sites oriented along a nw striking depth transect these records represent the first high resolution carbon isotope records across oae 2 from the south american margin of the tropical atlantic due to the scarcity of age significant fossils the main purpose of this study was to develop a detailed carbon isotope stratigraphy in order to correlate the ctbi across the depth transect and to tie this to biostratigraphically well defined sections in the western interior basin pueblo usa boreal shelf seas eastbourne england and western tethys oued mellegue tunisia all four sections studied document a 6 increase of 13corg values at the base of the ctbi which is followed by an interval of elevated 13corg values and a subsequent decrease our results supply an important stratigraphic base for subsequent paleoceanographic studies on late cenomanian to early turonian sediments from demerara rise and elsewhere

fault interaction may generate multiple slip vectors on a single fault surface slip vectors of multiple orientations on a single fault surface may be explained by the interaction of crustal scale faults in response to nearby earthquakes field observations of at least five sets of striae on a single planar fault surface can be reproduced in numerical experiments these experimental results suggest that slip vectors having variable orientations may reflect a highly transient local stress state in an otherwise constant regional stress field our results suggest that stress inversion techniques should be applied with caution to areas with complex regional scale fault patterns

tectonic controls on the yamanlar volcano and yuntdagi volcanic region western turkey implications for an incremental deformation abstract over the past ten years it has been proposed that the western part of the menderes massif was strongly structurally controlled by the izmir balikesir transfer zone ibtz yamanlar volcano is a key area for understanding the deformation of miocene volcanoes in western turkey because of its progressive extensional tectonics structural analysis provides that this volcano has undergone the incremental tectonic controls in western turkey since early miocene the volcano experienced deformation and erosional processes associated with activity of intense tectonic regime that resulted in the dissection of the southern flank of the volcano mostly by ne sw striking oblique and strike slip faults together with cross cutting faults during and after miocene period the orientation of volcanic domes dykes and intrusive bodies indicates successive and reactive tectonic phases that caused incremental complex movements of numerous fault blocks during the destruction area of the yamanlar volcano structural data documents that the region around izmir bay and yuntdagi volcanic complex is prevailed by far more than three deformation phases cross faults do not reflect to the ne sw trending transtensional deformation thereafter early middle miocene for just the area of the yuntdagi volcanic system the nearly e w oriented faults are inferred to be the most likely cause of the destruction of both the yamanlar and dumanlidag volcanoes coincident with the last stage of the deformation phases the region experienced this study proposed that the western part of the mmcc was affected by different deformational phases instead of ibtz resulting in different stage magma ascents i early miocene transtensional tectonic ii middle miocene extensional tectonic related to slab retreat and iii late miocene tectonic block rotation

the role of crustal heterogeneity in controlling vertical coupling during laramide shortening and the development of the caribbean north america transform boundary in southern mexico insights from analogue models analogue models of polyphase deformation involving crustal differences in strength thickness and density give insights into lateral and vertical strain propagation during late cretaceous shortening and early tertiary left lateral shearing related to the early development of the north america caribbean plate boundary in southern mexico analogue models reproduce a two phase deformation characterized by a first stage of compression orthogonal to the plate boundary simulating deformation induced by the laramide orogeny followed by a later stage of left lateral transpression associated with the transfer of the chortis block from the north american to the caribbean plate during the early stage of development of the new plate boundary in early tertiary times based on detailed structural observations in the guerrero morelos platform and the western part of the mixteco terrane of southern mexico we document that a transpressive regime affected continental red bed sequences of early paleocene to late eocene and rotated and refolded laramide structures during this second phase our model ends before the transtensional regime that affected the region which is marked by a volcanic episode of late eocene oligocene this change in the deformation regime records the passage of the nw tip of the chortis block north america cocos caribbean triple junction when subduction replaced transform faulting along the southern mexico margin the models focus on the structures formed around the flanks of a thicker more rigid crustal block that simulates the rock assemblages of the palaeozoic orogens of southern mexico mixteco oaxaca juarez block mojb the comparison of the mechanism of defor mation of three different analogue models with the natural prototype explains most of the structures observed around the mojb counterclockwise vertical axis rotations of pre existing structures in the western flank of the mojb observed in the guerrero morelos platform are consistent with the modelled structures vertical movements of the modelled mojb induced by the transpressive regime can explain the papalutla thrust and the basement upheaval and gravitational sliding of the cover in the tentzo ranges observed at the western and northern margins of the mojb respectively the growth and propagation of thrusting controlled by the geometry of the block along the eastern margin also correlates with the vista hermosa fault the propagation of strain to the north increases with higher contrast in strength of the thick block with respect to the adjacent modelled crust analogue modelling failed to reproduce all the structural details of southern mexico and specifically the structures observed inside the mojb the latter however are controlled by pre existing discontinuities which are not simulated in the model as a whole the results demonstrate that crustal heterogeneity in a developing left lateral plate boundary zone produces a stronger vertical coupling between ductile and brittle crust and a widening of the defor mation zone along the margin of the north america plate in southern mexico

kinematic setting and structural control of arc volcanism abstract the evaluation of the kinematic setting and the structural control of volcanic arcs are important in defining the tectono magmatic processes along convergent plate boundaries however our knowledge is fragmented and the available data highlight different behaviours this study analyzes the kinematic setting the structural control and the volcanic productivity of 16 arcs these arcs are characterized by predominant extensional compressional strike slip or oblique motions there is an overall coupling between the normal vs parallel motions along the arc and those of the underlying slab therefore the higher the trench normal or parallel component of the subduction rate the higher is the amount of arc normal or parallel motion this relation confirms that strain partitioning is in general feasible at many convergent settings involving also the volcanic arc portion the arc normal motion may be characterized by extension or compression as a function of the increase in the trench normal convergence or subduction velocity the lack of an evident relation between the subduction rate or convergence rate and the volcanic productivity of the arc is partly inconsistent with previous studies which highlight a relation between the subduction rate and melt production below arcs this discrepancy mainly suggests that different processes control the generation rise and eruption of magma at different depths varying the intrusive extrusive ratio along arcs the structure of the arc does not control the distribution of the volcanoes however it does control the volcanic output through different processes even though regional or local extension associated with strike slip or compressive structures is the ultimate requisite in any setting in general the higher the amount of extension the higher is the output rate along the arc

cretaceous eocene magmatism and laramide deformation in southwestern mexico no role for terrane accretion in southwestern mexico late cretaceous to early tertiary deformation has been generally associated with the laramide orogeny of the cordillera several alternative models consider the deformation to result from the accretion of the guerrero terrane formed by the zihuatanejo arcelia and teloloapan intraoceanic island arcs to the continental margin of the north american plate here we present a detailed geologic and structural study and new 40 ar 39 ar and u pb ages for a broad region in the centraleastern part of the guerrero terrane that allow the accretion model to be tested in the huetamo ciudad altamirano part of the region an almost complete cretaceouspaleogene succession records the transition from an early cretaceous shallow marine environment to continental conditions that began in santonian times followed by the development of a major continental eocene magmatic arc folding of the marine and transitional successions signifi es a shortening episode between the late cenomanian and the santonian and a subsequent out of sequence coaxial refolding event in maastrichtian paleocene time amplifi ed the previous structures a major left lateral shear zone postdates the contractional deformation and it passively controlled the geographic distribution of eocene silicic volcanism minor transcurrent faulting followed

tectono stratigraphic terranes and mineral resource distributions in mexico about 80 of the southern part of the north american cordillera within the republic of mexico is made up of suspect terranes these terranes are suspect because their paleogeographic setting with respect to cratonic north america at various times through much of phanerozoic time is uncertain much of northeastern and southeastern mexico is underlain by basement accreted during late paleozoic time an extension of the appalachian ouachita orogeny this orogen has been considerably modified by jurassic strike slip translations related to the opening of the gulf of mexico western and southwestern mexico is largely made up of several distinct but coeval latest jurassic to late cretaceous submarine magmatic arc terranes with unknown basement that appear to have accreted against the disrupted north american margin by early tertiary time only northeastern sonora and the state of chihuahua appear to be floored by unmoved north american cratonic basement the combined effect of mesozoic accretions and translatio

analogue models of an early cenozoic transpressive regime in southern mexico implications on the evolution of the xolapa complex and the north american caribbean plate boundary

organic rich and manganese sedimentation during the cenomanian turonian boundary event in the outer carpathian basins a new record from the skole nappe poland abstract upper cenomanian lower turonian strata in the splawa section of the skole nappe outer carpathians poland provide a detailed palaeoenvironmental record of the oceanic anoxic event oae 2 from a deep water basin below the ccd at the northern margin of the western tethys the oae 2 succession consists of black parallel laminated radiolarian shales enriched in organic matter of mainly marine origin toc up to 8 organic carbon isotope variations together with microfossil datum events allowed to correlate the studied organic rich facies with other black shale successions along the outer carpathians and other areas of the western tethys the mean values of sedimentation rate 3 5 5 5 mm kyr and organic matter accumulation rate 0 5 g m2 year calculated for the oae 2 interval in the outer carpathian basins are comparable to the values from other deep water marginal basins in the western tethys the average redox sensitive trace metal contents and their enrichment factors indicate water column anoxia in the outer carpathian basins intensified during the interval of the largest shift in 13c values the periods of bottom anoxia were interrupted by intervals of variable eh conditions with sedimentation of hemipelagic green shales including poor agglutinated foraminiferal assemblages an increase in productivity of organic and siliceous plankton during the oae 2 is documented based on increased values of silica and barium enrichments this closely corresponds to an interval of the largest positive stable carbon isotope excursion related to the beginning of the largest sea level rise linked with intensive coastal upwelling extremely low sedimentation rate or even hiatuses and the increase in deep water circulation causing basin oxygenation resulted in precipitation of two ferromanganese layers their formation was related to changes in hydrodynamic conditions at the basin floor expressed by precipitation of fe mn carbonates some chemical indices seem to suggest that the primary metal enrichments could be related to hydrothermal activity however the concentrations of some elements show also on hydrogenic to diagenetic influences in these metalliferous sediments the precipitation of these fe mn sediments was probably synchronous with the manganese enrichment across the c t transition recorded in other western tethyan basins and in the adjacent epicontinental seas

slickenside kinematic indicators abstract a new classification of slickenside kinematic indicators is presented based on 61 criteria these slickensides have been subdivided into eleven major groups v or crescentic markings steps fractures trains of inclined planar structures trailed material asymmetric elevations deformed elements mineralogical crystallographic orientations asymmetric plan view features asymmetric cavities and asymmetric folds this classification constitutes a useful tool for geologists interested in the determination of the shear sense in fault surfaces bearing slickensides examples of application of this classification to natural fault surfaces at different scales are presented

tectonic stress controls on ascent and emplacement of magmas abstract the tectonic stresses can significantly affect the propagation of a magma filled crack it has been pointed out that the rheological boundaries control the emplacement of magmas through the effect of stress however it has not been clarified how the role of rheological boundaries depends on the regional tectonic and thermal states we have evaluated the role of rheological boundaries under various tectonic and thermal conditions and found that the level of magma emplacement may jump according to the changes in the tectonic force or the surface heat flow the stress profiles were estimated by a simple model of lithospheric deformation we employed a three layer model of the lithosphere the upper crust the lower crust and the upper mantle have different rheological properties a constant horizontal force is applied to the lithosphere and the horizontal strain is assumed to be independent of depth when realistic tectonic forces 10 11 n m are applied the rheological boundaries mainly control the emplacement of magma the emplacement is expected at the moho the upper lower crust boundary and the brittle ductile boundary for lower tectonic forces 11 n m the tectonic stress no longer plays an important role in the emplacement of magmas when the tectonic stress controls the emplacement the roles of rheological boundaries strongly depend on the surface heat flow when the surface heat flow is relatively high 80 mw m 2 the stress in the mantle is quite low and the moho cannot trap ascending magmas for relatively low heat flow 2 on the other hand the moho acts as a magma trap and the upper lower crust boundary acts as a magma trap only when the magma supply rate is sufficiently high our results suggest that the emplacement depth can change responding to the change in the tectonic force and or that in the surface heat flow this may provide us a key to understand the relation between the evolution of a volcanic region and its tectonic and or thermal history

granitic magma transport by fracture propagation granitic magmas commonly ascend tens of kilometres from their source terranes to upper crustal emplacement levels or to the earth s surface apart from its obvious bearing on the interpretation of the geology and geochemistry of felsic igneous rocks the magma ascent mechanism critically affects any modelling of the metamorphic evolution of the upper lithosphere as well as its rheology we propose that in general granitic magmas ascend via propagating fractures as dykes in extremely short time periods long distance diapiric transport of granitoid magmas through crustal sections is not viable on thermal or mechanical grounds and there is an apparent total absence of field evidence for diapiric rise of such magmas even in supposed type localities neither the shapes nor internal or external characteristics of granitic plutons necessarily reveal anything about the transport of their precursor magmas these are purely arrival phenomena dictated by local structure kinematics and stress states based on existing numerical treatments of the problem we show that granitic magmas are apparently sufficiently inviscid to travel through fractures to high crustal levels without suffering thermal death by freezing for example a 2000 km3 granitic batholith can be inflated by a 1 km 3m 20 km deep dyke system in less than 900 years the model proposed has numerous far reaching petrological and rheological consequences some of which are outlined

the arcelia graben new evidence for oligocene basin and range extension in southern mexico basin and range extension which began in the tertiary and continues today is well documented in mexico north of the trans mexican volcanic belt in contrast evidence for basin and range extension in southern mexico is largely limited to the oaxaca basin a north northwest trending miocene graben we discovered another north northwest trending tertiary basin the arcelia graben approximately 200 km west of the oaxaca basin and 50 km south of the trans mexican volcanic belt arcelia graben subsidence began in the early tertiary and mostly ended prior to accumulation of upper oligocene volcanic rocks indicating basin and range extension in this area was limited to a 32 m y interval in the tertiary extension near arcelia is among the oldest basin and range style deformation documented in mexico most subsidence in the arcelia graben preceded onset of oaxaca basin subsidence by at least 8 m y this suggests the intriguing possibility of eastward migration of basin and range extension in southern mexico during the middle tertiary

the exhumed eocene sultepec goleta volcanic center of southern mexico record of partial collapse and ignimbritic volcanism fed by wide pyroclastic dike complexes the sultepec goleta area in southern mexico hosts one of the major eocene silicic volcanic centers that make up the cenozoic volcanic record of the north central part of the sierra madre del sur this center is represented by a partially exhumed nne ssw trending ignimbritic field that covers an area of 400 km2 with a preserved volume of 200 km3 a remarkable feature of volcanic center is the exposure of pyroclastic dike complexes up to 1 km wide that extend almost continuously along the western and southern flanks of the volcanic field the ignimbritic record comprises four units with different textural features as well as variable proportions of the main components i e phenocrysts pumice and lithic clasts the most extensive unit is the basal goleta ignimbrite of which the middle and upper members are richer in phenocrysts than the lower member it has a thickness that ranges from 200 m in the north to at least 600 m in the south of the volcanic center the overlying ignimbrite units cienega lobera and potrero ignimbrites are phenocryst poor and are distributed in the northern sector of the volcanic field the semi curvilinear trend of the pyroclastic dikes along the southern flank of the goleta range coupled with the greater thickness of the goleta ignimbrite encircled by the dikes is indicative of the development of a partial collapse caldera based on spatial relationships as well as analogies in the nature and abundances of the components it has been recognized that the central and southern dike complexes fed the relatively phenocryst rich goleta ignimbrite whereas the phenocryst poor units of the northern sector were extruded through the pyroclastic conduits distributed around sultepec we suggest that the stirring produced by the partial collapse of the magma chamber roof beneath the southern sector of the study area triggered the tapping of phenocryst rich portions of the zoned magma chamber resulting in an increase of phenocryst content in the upper members of the goleta ignimbrite the presence of breccias and lithic rich facies dominated by wallrock fragments at the borders of the pyroclastic dikes including those of the northern extra caldera sector indicate that erosion of the conduit walls above the fragmentation level contributed to widening of the conduits and the construction of wide dike complexes

paleomagnetism of the cretaceous morelos and mezcala formations southern mexico a paleomagnetic study of platform facies carbonate rocks of the mid cretaceous morelos formation and deep water carbonate rocks of the overlying upper cretaceous mezcala formation sampled at zopilote canyon in guerrero state southern mexico indicates that their characteristic magnetization was acquired contemporaneously with folding of these rocks during the late cretaceous laramide orogeny the remanence carrier is interpreted to be magnetite although other mineral phases of high coercivity carry recent secondary overprints the overall mean is of dec 323 1j and inc 36 5j k 162 7 a95 2 7j n 18 sites 64 unfolding comparison with the north america reference direction indicates that this area has experienced a small yet statistically significant counterclockwise direction of 19 2f4 0j similar rotations are documented in other localities from southern mexico rotations are linked to mid tertiary deformation associated with the left lateral strike slip fault system that accommodated motion of the chortis and xolapa blocks d 2002 elsevier science b v all rights reserved

strike slip faults and k alkaline volcanism at el chichon volcano southeastern mexico abstract el chichon volcano is located in the northern part of the state of chiapas in southeastern mexico the volcano is situated within the transcurrent fault province characterized by a series of rising and sinking blocks bounded by sinistral strike slip faults the basement around el chichon is a sequence of jurassic evaporates cretaceous limestones and tertiary terrigenous rocks folded into open and an echelon structures the main structures in the area are the la union and caimba anticlines and the buena vista syncline the region is cross cut by two conjugate fault systems a dextral strike slip n s trending set and a sinistral strike slip e w set the most significant fault of the latter system is the san juan fault which controlled the emplacement of a basaltic dike near the village of chapultenango some 1 1 ma ago and the activity of el chichon during the last 0 2 ma additionally a series of n45 e trending faults chapultengo fault system have produced a half graben geometry of blocks on top of which el chichon has been emplaced microstructures such as slickensides tension fractures layer parallel slip stylolites and macro and meso folds of the sedimentary rocks indicate that during the late miocene the el chichon area was affected by a maximum principal stress 1 oriented n70 e a minimum principal stress 3 oriented n20 w and a vertical intermediate principal stress this stress pattern indicates that the area underwent a strike slip motion that produced widespread deformation the occurrence of crustal earthquakes

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

the dynamic history of the trans mexican volcanic belt and the mexico subduction zone abstract the trans mexican volcanic belt tmvb is a 1000 km long neogene continental arc showing a large variation in composition and volcanic style and an intra arc extensional tectonics it overlies the rivera and cocos slabs which display marked changes in geometry geophysical studies indicate that lithospheric mantle is very thin or absent beneath the forearc and arc the fluids from the slab are released in a 40 to 100 km wide belt beneath the frontal part of the arc and the lower crust beneath the arc is partially molten east of 101 w the tmvb is built on a precambrian to paleozoic crust with thickness of 50 55 km west of 101 w the tmvb is underlain by jurassic to cenozoic marine and continental arcs with a 35 40 km thick crust the evolution of the tmvb occurred in four stages 1 from 20 to 10 ma the initial andesitic arc moved inland showing progressively drier melting and eventually slab melting suggesting flattening of the subducted slab 2 since 11 ma a pulse of mafic volcanism migrated from west to east reaching the gulf of mexico by 7 ma this mafic lavas marks the lateral propagation of a slab tear triggered by cessation of subduction beneath baja california 3 thereafter the volcanic front started moving trenchward with a marked phase of silicic volcanism between 7 5 and 3 ma local emplacement of small volume intraplate like basalts since 5 ma and development of extensional faulting these features are related to slab rollback enhancing asthenophere flux into the mantle wedge and promoting partial melting of the crust 4 the modern arc consists of a frontal belt dominated by flux and slab melting and a rear belt characterized by more differentiated rocks or by mafic lavas with little or no evidence of subduction fluids but higher asthenosphere fingerprint

structure composition and age of the small islands of santa luzia branco and raso cape verde archipelago abstract the northern alignment of cape verde archipelago contains three small islands in its central sector santa luzia branco and raso on which there is very scarce geological petrological or geochronological information available the three islands along with the island of sao vicente arise on a continuous underwater relief less than 50 m deep and about 70 km long that formed from separate volcanic edifices the southernmost island raso is formed by accumulation of several tens of lava flows and some remaining strombolian cones an erosion surface covered by sediments allows the separation of two different episodes of growth the lower one older than 2 ma and the upper one between 0 9 and 1 2 ma the smaller island of branco is the remnant of a large stratovolcano about 6 km in radius 1000 m of altitude and between 5 and 6 ma in age santa luzia the northernmost and largest island of this particular group is essentially a remaining rift zone between 4 and 7 ma old at least 600 m high about 10 km long and 7 km wide the se extreme of the island is occupied by rocks of an older partially submarine stage of growth which also include gabbroic intrusions crossed by a dense network of dikes whose age may be estimated in 8 ma or more on the contrary the nw end shows the only remains of late post erosion activity that is of much less volumetric importance and it has been dated between 0 8 and 1 4 ma the three islands as a whole are formed by moderately or strongly alkaline basic and ultrabasic rocks similar to those found in the neighboring island of sao vicente ultraalkaline rocks only appear in raso and differentiate types as trachyandesites and phonolites are exclusively found in santa luzia the similar age found for the main stages of growth recorded in the mentioned four islands and even more in the other two islands santo antao and sao nicolau situated at the end of the barlovento alignment are not easily comprehensible with evolution models implying a simple spatial progression of the volcanic activity from east to west in this array of the archipelago on the other hand the temporal coincidence in periods of quiescence observed in the different islands seems to point to the existence of a cyclical activity for the ensemble

volcanic evolution of sao vicente cape verde islands the praia grande landslide abstract the island of sao vicente has undergone continuous volcanic activity from pliocene to pleistocene times the earliest evidence of activity corresponds to some 9 million years ago when a submarine edifice of alkaline basaltic affinity had already started to develop the island resulted from the growth of a single major edifice the sao vicente edifice built up in several distinguishable growth stages the early main stage occurred at 6 5 to 4 5 ma giving rise to an edifice of about 10 12 km in radius and more than 2500 m in height whose centre was located south of the present city of mindelo although the edifice in its earlier phase showed typical characteristics of a shield volcano the mindelo formation it adopted later on those of an ordinary composite volcano the madeiral monte cara formation a giant landslide event the praia grande landslide destroyed the ne sector of the edifice and left a 10 12 km large depression that was successively refilled by nephelinitic lava flows and some associated carbonatites the monte verde formation 4 5 3 1 ma the volcanic activity ceased about 3 2 ma ago when the edifice started to be deeply eroded until disappearing about 90 of its total volume only in recent times 0 3 ma very scarce and local strombolian activity has been developed aside the sao vicente edifice in the eastern sector of the island

origin and tectonic interpretation of multiple fault patterns abstract it is shown that in two dimensional and three dimensional deformation accommodated by fracture the symmetry of the fault patterns is an intrinsic attribute because it reflects the symmetry of either stress or strain tensors the deformation accommodated by sliding along pre existing planes when there is kinematic interaction between that planes forms multiple fault pattern and multiple slickenline sets during a single deformation event these fault patterns have no restrictions with respect to symmetry number of fault sets or fault orientation the kinematic analysis developed here shows that an interacting system is formed by two cross cutting faults and three slickenlines one slickenline must be parallel to the intersection line between the planes also it is demonstrated that the slickenlines generally do not correspond to the shear stress solution on the planes thus the interaction between planes does not satisfy the assumption of parallelism between shear stress and slip vector we conclude that the inversion methods to calculate paleostress tensors can lead to erroneous interpretations in structurally complex zones with many pre existing planes of weakness we propose four possibilities to form multiple fault patterns 1 two or more events of faulting obeying coulomb s law with a change of orientation of the principal stresses in each event 2 reactivation of non interacting planes according to the bott 1959 model 3 one three dimensional strain event that obeys the slip model this mechanism will form an orthorhombic four fault pattern and two slickenline sets in a single strain event and 4 one or more events obeying the interacting block model proposed here with or without rotation of the principal stresses we propose the last origin as the most common in continental regions

a major resurgent caldera in southern mexico the source of the late eocene tilzapotla ignimbrite the tilzapotla caldera constitutes the first discovery of a major tertiary collapse volcanic structure south of the mexican volcanic belt although it is spatially associated with silicic ignimbrites in a region relatively distant from the extensive ignimbritic province of the sierra madre occidental smo it is among the largest collapse calderas documented in mexico the caldera is defined by a 3324 km semi elliptical structure that encircles the largest exposures of the tilzapotla ignimbrite and corresponds to the structural margin rather than the topographic rim a central uplifted block limited by nw trending faults is the main indication of a resurgent stage the caldera structural margin is surrounded by extensive exposures of cretaceous marine sequences that structurally define a broad elliptical dome 4535 km originated in the first stage of the caldera evolution there is evidence showing that the 34 ma tilzapotla ignimbrite represents the climatic event of the caldera collapse it is constituted by a massive sequence of crystal vitric tuff with conspicuous euhedral biotite and abundant quartz the intra caldera facies isintercalated withmega andmeso breccias oflimestoneandanhydrite fragments derivedfromthe slumpingofthe caldera wall during the caldera collapse the overlying sequence includes post collapse ignimbrites as well as amphibole and pyroxene bearing dacitic to andesitic lava flows the age 33 to 32 ma and isotopic signatures of these lava flows indicate a resurgent event related with the input of more primitive magmas into the magma chamber the rectilinear northeastern and southwestern segments of the structural margin of the caldera correspond to nw trending tectonic lineaments that are part of a regional strike slip system active at the time of the caldera formation we interpret that the nw tectonic structures defined zones of weakness that accommodated the caldera collapse in the northeastern and southwestern segments of the caldera structural margin d 2004 elsevier b v all rights reserved

cenozoic magmatism of the sierra madre del sur and tectonic truncation of the pacific margin of southern mexico abstract in southern mexico the sierra madre del sur sms preserves the record of a long lived magmatic arc that vanished diachronically during paleogene early miocene time decreasing in age to the southeast the magmatic record consists of a coastal plutonic belt dominated by granites and granodiorites with minor mafic and intermediate intrusions and an inland volcanic belt dominated by intermediate and silicic volcanic centers the juxtaposition of the plutonic belt with the trench indicates the removal of most of the paleogene forearc during the oligocene abundant evidence indicates that subduction erosion played a significant role in the reconfiguration of the continental margin of southern mexico following cessation of magmatism this is evident in the subsidence of continental segments contiguous to the present trench and the loss of lower continental crust as inferred from the present shape of the wedge as well as in the estimations of the former crustal thickness based on geobarometry of the exposed levels of plutonic complexes the late cretaceous cenozoic decreasing age trend in the sms shows three time intervals in which magmatism ceased at different rates the general southeastward younging trend of magmatism as well as the left lateral shear zones of the central and coastal zones of the sierra madre del sur in combination with constraints imposed by the prevalent model of the caribbean plate evolution have been invoked to consider the chortis block as the removed segment during margin truncation the main drawbacks of this model are the mismatch in the chronology of migmatitic complexes of northern chortis and the xolapa terrane in the continental margin of southern mexico and the presence of the late cretaceous to present unperturbed marine sequence in the gulf of tehuantepec in the assumed trajectory of the chortis block these considerations and the subduction erosion evidence suggest a more complex scenario than having chortis juxtaposed immediately adjacent to the present day margin there are alternative possibilities to explain removal of the forearc ranging from a western provenance for the chortis block as has been suggested in other models to intermediate positions for chortis with a more southern trajectory with respect to the present day trench such a trajectory would leave a broad zone with shear strain structures more susceptible to removal by subduction erosion after the passage of the trench trench transform triple junction this would reconcile the evidence of subduction erosion and the indications of the northwestern provenance of the chortis block

latest cretaceous to miocene deformation events in the eastern sierra madre del sur mexico inferred from the geometry and age of major structures the cretaceous cenozoic major lithologic units and structures of the sierra madre del sur are well known the laramide orogeny is generally considered as the cause of the contractile structures but the details about the migration kinematics and intensity of deformation are poorly known furthermore the deformation events responsible for the post laramide strike slip and normal faults have not been identified in this paper we document the migration of the deformation events that occurred in southern mexico from maastrichtian to miocene time we identify different groups of structures representing three successive deformation events based on the geometry age and kinematics of tectonic structures deformation migrated from west to east the first event corresponding to the laramide orogeny occurred during late cretaceous time in the guerrero morelos platform and ended in the middle eocene in the east within the veracruz basin the oaxacan fault system which bounds the acatlan oaxacan block to the east records laramide shortening from six structural sections we interpret the juxtaposition of the oaxacan complex against the mylonite belt of the sierra de juarez with subsequent uplift of the eastern border of the oaxacan complex and finally the gravitational overriding of the sedimentary cover in a radial centripetal arrangement the second event produced strike slip faulting during ne sw horizontal shortening from eocene to oligocene time the third event produced normal and strike slip faults indicating ne sw horizontal extension during oligocene miocene time major structures produced during these three deformation events are roughly distributed in an arcuate pattern bounding the block formed by the acatlan and oaxacan complexes based on this pattern and the relatively less deformed mesozoic rocks within the acatlan oaxacan block we interpret that most of the deformation resulted from the impingement of this block on thinner crustal domains adjoining the block

rhyolitic volcanism in extension zone associated with strike slip tectonics in the taxco region southern mexico abstract the taxco volcanic field tvf is part of a broad magmatic province in southern mexico it constitutes an isolated zone of deeply dissected volcanic rocks encircled by outcrops of mesozoic sedimentary and volcano sedimentary units a thick unit of rhyolitic lava flows associated with domes and at least two ignimbrite units forms the tvf this volcanic sequence is distributed within a well defined zone it overlies and is in part contemporaneous with continental sedimentary beds limited by major faults geochronologic data indicate that most rhyolitic volcanism in the area is oligocene in age and synchronous with episodes of strike slip faulting we document two successive phases of strike slip faulting for the late eocene early oligocene interval the first with nnw extension and the second with ne extension in both cases pre existing structures were reactivated and sedimentary basins were developed in response to displacement along major faults the stratigraphic sequence gives evidence that the tvf is located in an extensional basin associated to strike slip faults the evolution of the basin underwent a change from sedimentary deposition with subsidence to piling up by volcanism the result of this change was the development of a volcanic pile with elevations higher than the surrounding mesozoic rocks according to the fault kinematics stratigraphy and the volume of volcanic rocks the rhyolitic volcanism was emplaced in the area of maximum extension showing that magma flowed into low pressure zones the small number of faults within the oligocene volcanic sequence suggests that volcanism inhibited normal faulting and that magma partially filled the space generated in the extended zone produced by the strike slip faulting

boiling and depth calculations in active and fossil hydrothermal systems a comparative approach based on fluid inclusion case studies from mexico abstract a boiling model that considers the increase of salinity due to the steam loss and uses a combined density of the coexisting vapor and liquid phases was applied to fluid inclusion data from los azufres geothermal zone and from an eocene epithermal vein of taxco these case studies are taken as examples of active and fossil hydrothermal systems respectively in los azufres high temperatures of homogenization 300 c are commonly attained at depths between 1500 and 2000 m whereas salinity values above 2 0 wt nacl eq occur within the upper 500 m of the system suggesting that the geothermal zone is largely affected by boiling the depths calculated with the boiling model are close to real depths with accuracy greater than 99 for one case however considerably large error 30 was obtained to the top of the geothermal system due to enhanced co2 concentrations contrastingly the depths estimated by plotting microthermometric data on boiling point curves of constant salinity and discarding the effect of vapor on hydrostatic pressure were systematically shallower than real ones implying an underestimation of depth of up to 50 for the application case of the taxco epithermal deposit microthermometric data describe a boiling evolution path in the temperature salinity space although some values deviate from it thus likely reflecting local mixing with fluids of contrasting salinity according to our model boiling occurred from a paleo depth of 360 m which corresponds to a current sampling depth of about 200 m this level in the hydrothermal system coincides with the boundary between a lower base metal zone and an upper silver rich zone these results suggest that the descriptive models for epithermal deposits could be incorrectly calibrated in terms of depth therefore they could be revised and corrected by applying the boiling model used in this paper

magmatic processes that generate chemically distinct silicic magmas in nw costa rica and the evolution of juvenile continental crust in oceanic arcs northwestern costa rica is built upon an oceanic plateau that has developed chemical and geophysical characteristics of the upper continental crust a major factor in converting the oceanic plateau to continental crust is the production evolution and emplacement of silicic magmas in costa rica the caribbean large igneous province clip forms the overriding plate in the subduction of the cocos plate a process that has occurred for at least the last 25 my igneous rocks in costa rica older than about 8 ma have chemical compositions typical of ocean island basalts and intra oceanic arcs in contrast younger igneous deposits contain abundant silicic rocks which are significantly enriched in sio2 alkalis and light rare earth elements and are geochemically similar to the average upper continental crust geophysical evidence high vp seismic velocities also indicates a relatively thick 40 km addition of evolved igneous rocks to the clip the silicic deposits of nw costa rica occur in two major compositional groups a high ti and a low ti group with no overlap between the two the major and trace element characteristics of these groups are consistent with these magmas being derived from liquids that were extracted from crystal mushes either produced by crystallization or by partial melting of plutons near their solidi in relative terms the high ti silicic liquids were extracted from a hot dry crystal mush with low oxygen fugacity where plagioclase and pyroxene were the dominant phases crystallizing along with lesser amounts of hornblende in contrast the low ti silicic liquids were extracted from a cool wet crystal mush with high oxygen fugacity where plagioclase and amphibole were the dominant phases crystallizing the hot dry reducing magmas dominate the older sequence but the youngest sequence contains only magmas from the cold wet oxidized group silicic volcanic deposits from other oceanic arcs e g izu bonin marianas have chemical characteristics distinctly different from continental crust whereas the nw costa rican silicic deposits have chemical characteristics nearly identical to the upper continental crust the transition in nw costa rica from mafic oceanic arc and intra oceanic magma to felsic upper continental crust type magma is governed by a combination of several important factors that may be absent in other arc settings 1 thermal maturation of the thick caribbean plateau 2 regional or local crustal extension and 3 establishment of an upper crustal reservoir

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

indium and yttrium in north atlantic and mediterranean waters comparison to the pacific data abstract vertical profiles of dissolved indium and yttrium were determined in the eastern north atlantic and the mediterranean sea to compare with those of the north pacific reported earlier the y concentrations in the surface waters are 120 pmol kg in the north atlantic and 205 pmol kg in the mediterranean sea which are significantly higher than 80 pmol kg in the north pacific the difference may be attributable to the different strength of input of y to the oceans from fluvial and aeolian sources in contrast the deep water concentration of y increases in the order of north atlantic north north pacific exhibits an inverse trend of y and nutrients indium is highly particle reactive 47 association in the mediterranean sea and must have a short mean oceanic residence time however the featureless dissolved in profile in the mediterranean sea is clearly different from the profiles of dissolved al showing increase with depth hydes et al 1988 measures and edmond 1988 suggesting that significant fractionation of the two elements is taking place in the ocean the interoceanic variations of dissolved in and al may be ascribed to the different intensities of external input of which aeolian has been considered to be major rather than fluvial however the difference of in and al concentrations in the deep waters of the above oceanic basins are significantly greater than those of other refractory elements such as ce ti hf and zr whose major sources to the ocean are also considered to be aeolian furthermore the in al ratios in seawater are about two orders of magnitude greater than the average crustal ratio thus some additional sources though not yet certain may be required to explain the high concentrations of in in the atlantic and the mediterranean deep waters

ages of epithermal deposits in mexico regional significance and links with the evolution of tertiary volcanism we present an analysis leading to a new space time classification of the mexican epithermal deposits based on new data and interpretations from seven deposits in central mexico and a new age for the la guitarra deposit temascaltepec district from the latter adularia from a barren stage of mineralization and from quartz bands of the main ore stage together with orthoclase phenocrysts from host monzogranites with a known late laramide age were analyzed by the 40ar 39ar method the ages obtained from adularia samples are 32 9 0 1 ma and 33 3 0 1 ma corresponding to the middle part of lower oligocene these ages are slightly younger than that of the ignimbrite succession exposed next to the study area and they suggest that the deposit is related to the crystallization of comagmatic intrusive rocks contrary to previous assumptions of a miocene age for several epithermal deposits in southern mexico the ages from la guitarra show that south of the trans mexican volcanic belt these deposits may be oligocene or older in addition the interpretation of a genetic link between intrusive rocks and epithermal mineralization is consistent with previous work on fluid geochemistry of the deposit to evaluate the obtained ages in a regional context we analyzed the geographic distribution and age of tertiary epithermal deposits in central mexico our study shows that mineral deposits coincide with the main volcanic pulses of the sierra madre occidental the distribution of epithermal deposits can be thus divided into three main groups 1 between 48 and 40 ma 2 between 36 and 27 ma and 3 between 23 and 18 ma within the first group are the batopilas and topia deposits that record the oldest ages found for epithermal deposits in mexico deposits of this age are hosted by the lower volcanic supergroup of the sierra madre occidental and are related to the laramide magmatism of the sierra madre occidental many of the epithermal deposits in mexico occur within the second time span in a northwest southeast belt from chihuahua to the mexico and guerrero states these deposits are related to the main episode of ignimbrite flare up of the sierra madre occidental the third group includes the bolanos san martin de bolanos and pachuca real del monte deposits plus possibly seven other epithermal deposits analyzed in this work these deposits are related to the last ignimbrite flare up of the sierra madre occidental with a general west northwest east southeast distribution recently documented in the southern sierra madre occidental

a finite element model of the stress field in strike slip basins implications for the permian tectonics of the southern alps italy abstract a set of finite element models have been used to simulate pull apart basin formation related to a releasing overstep along two right strike slip faults the modelling results have shown the stress field at a releasing fault overstep with respect to strike slip transtensional and transpressional boundary constraints based on the assumption that normal faults strike parallel to the maximum horizontal stress we have predicted the orientation of potential normal faults within the overstep zone linear elastic rheology and plane stress conditions were assumed and four kinds of models with different overlap separation ratios 0 0 5 1 and 2 between the strike slip faults were analysed displacement boundary conditions were applied to the upper left hand and lower right hand corners of the models in order to simulate a right strike slip movement outward and inward displacement components were applied respectively to reproduce transtensional and transpressional conditions all the models mostly show an extensional stress field i e with at least one extensional principal stress axis large zones inside the overstep are affected by general extension both principal axes are extensional and their size increases with both an increase in the overlap separation ratio and with changes from transpressional to transtensional conditions the areas of maximum extension are located near the ends of the fault segments inside the overlap area it is fairly probable that the genesis of the extensional faults that bound the pull apart basins observed throughout the world occurs in these maximum extensional areas and that the extensional faults are parallel to the maximum compression stress axis trajectories the angle between the potential normal faults and the strike slip faults is strongly dependent on the overall boundary conditions i e pure strike slip transtension and transpression of the model in contrast the elastic rock properties and the overlap separation ratio of the faults have a minor control the angle between the normal and strike slip faults can vary from a few degrees in transtensional conditions to 100 120 in transpressional conditions inside the overlap zone an area of subsidence occurs the axis of which is coincident with the junction of the strike slip fault tips where it is also at a maximum i e in the same zones of maximum extension a comparison of the modelling results with the structural setting of the collio and pramollo permo carboniferous pull apart basins southern alps italy indicates that the collio basin developed under strike slip conditions while the pramollo basin was formed under transtensional conditions

the role of magma overpressure in suppressing earthquakes and topography worldwide examples in an extending terrane basaltic magma supplied at a pressure greater than the least principal stress overpressure may be capable of suppressing normal faulting and the earthquakes and topographic relief that commonly accompany normal faulting as vertical dikes intrude they press against their walls in the direction opposite the least principal stress and increase its magnitude the emplacement of tabular intrusions causes the internal magma pressure to act selectively in opposition to tectonic stresses this process tends to equalize the stresses and thus diminishes the deviatoric stress difference between maximum and minimum stresses that creates faults and causes earthquakes observations of the pattern of seismicity and magmatism worldwide indicate that magmatism commonly supplants large earthquakes as the primary mechanism for accommodating tectonic extension recognizing the extent of magmatic stress accommodation is important in assessing seismic and volcanic risks

lateral spreading of the middle to lower crust inferred from paleocene migmatites in the xolapa complex puerto escondido mexico gravitational collapse of a laramide orogen abstract field and laboratory studies including textural petrological and geochronological analysis of migmatites of the xolapa complex affected by the same deformational event suggests that the deformation occurred during crystallization of neosome the deformation represents sub horizontal spreading of middle to lower crust according to structural data and occurred during the paleocene as inferred from u pb laser ablation dating of zircon according to a compilation of stratigraphic magmatic and deformational data reported in southern mexico the sub horizontal spreading occurred after the culmination of the upper cretaceous laramide orogeny compiled regional data indicate a coast to coast mass transfer in the upper crust from zihuatanejo to veracruz from the cretaceous tertiary boundary until the mid eocene b contemporaneous adakite like magmatism and c extension in the mixteco and oaxaca terranes of southern mexico coeval to compression in the surrounding crust because all these observations are spatially associated to areas of the mixteco and oaxaca terranes we propose that these areas experienced a post orogenic gravitational collapse since the cretaceous tertiary boundary until the mid eocene this proposal implies that the gravitational collapse was recorded in the xolapa complex by migmatite formation in response to ascent of the brittle ductile transition decompression of the orogenic root and or shear driven asthenospheric upwelling gravitational collapse deformation was accommodated by the subsequent flow of the migmatites towards the foreland preceding the eastward escape of the chortis block

cenomanian palaeotemperatures derived from the oxygen isotopic composition of brachiopods and belemnites evaluation of cretaceous palaeotemperature proxies sixty low magnesium calcite samples from cenomanian articulate brachiopods belemnites and oysters from the epicontinental shelf sea of europe were geochemically and microscopically studied in order to evaluate their preservation and potential as carriers of palaeoenvironmental information the sampled localities in northern spain germany and southern england cover the transition between subtropical 25 30 n and temperate 33 38 n climates mean cenomanian salinity adjusted palaeotemperatures of diagenetically unaltered terebratulid and rhynchonellid brachiopods are 25 5 1 3 c in spain and 19 4 1 9 c in southern england the resulting low to mid latitude meridional temperature gradient of 0 7 c per degree latitude is similar to the gradient today and suggests modern heat transport values for cenomanian low latitudes oxygen isotopic composition of unaltered specimens of the upper cenomanian belemnite praeactinocamax plenus mean 0 2 is enriched in 18o by 1 in comparison to brachiopod shells correspondingly upper cenomanian palaeotemperatures derived from belemnite calcite 13 0 1 3 c underestimate modelled mid latitude sea surface temperatures by 6 c since p plenus occurs as boreal pulse fauna during a short interval in the late cenomanian its heavy oxygen isotopic composition can be attributed either to migration from a cooler and or deeper water mass or to unknown vital effects

dike transport of granitoid magmas thermal and fluid dynamical analyses suggest that for viscosities and density contrasts spanning the range considered typical for many calc alkalic granitoids dike ascent is a viable mechanism for the transport of large volumes of granitoid melt through the continental crust we present calculations showing that a granitoid melt with calculated viscosity of the order of 10 6 pa s and a density contrast between magma and crust of 200 kg m 3 can be transported 30 km through the crust in 1 month corresponding to a mean ascent velocity of 1 cm s using analysis modified from numerical studies of the flow of basaltic magmas in dikes we also present an expression that allows the calculation of the critical minimum dike or fault width required for granitic magma to ascend without freezing for all reasonable estimates of cordilleran granitoid viscosity and density contrast the critical dike width is determined to be between 2 and 7 m calculated peak batholith filling rates are orders of magnitude greater than mean cavity opening rates based on estimated fault slippage which is consistent with chemical evidence for intermittent supply of magma pulses